# Mumsnet Talk

## Why is my answer not correct?

(62 Posts)
Wed 30-Oct-13 10:02:05

24 pirates.

1/3 found gold

1/4 found rubies

1/2 found neither

How many found gold and rubies?

My calc was 8 found gold, 6 found rubies, 12 found nothing.

So, 14 found gold and rubies.....but 14 + 12 = 26........

Waaah

I love literacy but maths aint my thing.......

Wed 30-Oct-13 10:07:17

Bump

Seeline Wed 30-Oct-13 10:11:21

I came up with the same as you.... and I've got an A level (although that was a very long time ago ) I got one of DDs Y5 maths questions wrong hte other day too

Ahardyfool Wed 30-Oct-13 10:12:18

DS and I think there is a typo!

Wed 30-Oct-13 10:12:34

Lol Seeline! I need the correct answer though!

I think I am reading it wrong.....is there a small fraction that found gold AND rubies?

Wed 30-Oct-13 10:13:19

wouldn't it be 8? If only 8 had gold then no more than that can have gold and rubies? << hopeful >>

DrownedGirl Wed 30-Oct-13 10:13:49

Some of those who found gold also found rubies
That's the number you are looking for

There are only 12 who could have found gold or rubies

Where is the overlap

Not all of them who found gold found rubies as well

Wed 30-Oct-13 10:14:46

or maybe it would be 6?

Wed 30-Oct-13 10:14:52

Ds thinks the answer is 2........

FunnysInLaJardin Wed 30-Oct-13 10:14:58

I think your answer is right and that 2 pirates found both gold and rubies

YDdraigGoch Wed 30-Oct-13 10:15:02

Some of them might have found more than one thing. So the 8 that found gold might also have found rubies.

Ahardyfool Wed 30-Oct-13 10:15:07

I think the small fraction who found both could be the key here... This would make this a riddle more than a legitimate maths question!

Wed 30-Oct-13 10:16:46

24 pirates

12 found nothing therefore 12 found gold, rubies or both.

If 8 found gold and 6 found rubies then then 2 must have found both or there wouldn't be 12 who had found nothing.

So 6 just gold, 4 just rubies, 2 both =12

Helspopje Wed 30-Oct-13 10:16:55

12 none
6 r
8 g
6+8 = r + g +rg
14 = r + g + rg
rg = 2

Wed 30-Oct-13 10:17:22

8 found gold.
6 found rubies.
12 found nothing.

So 12 found SOMETHING.
12 - 8 who found gold = 4 who found ONLY rubies.
6 total rubies - 4 rubies only = 2 gold AND rubies.

Wed 30-Oct-13 10:17:29

I was adding together the pirates that found gold and the pirates that found rubies, not thinking there was a small percentage who found both.

Wed 30-Oct-13 10:17:56

Zombie thinks 2 pirates found both gold and rubies.

8 + 6 = 14 - 2 = 12 + 12 = 24

Wed 30-Oct-13 10:18:05

X-post with everyone

Wed 30-Oct-13 10:18:24

Gargh I type too slow one handed, baby DD2, I'm blaming you!

AliceinSlumberland Wed 30-Oct-13 10:18:34

The answer is 2 I think,

12 found nothing

8 found gold

6 found rubies

=26

But there are 24 in total so two pirates must have been counted in both the gold category and the rubies category, so they found both:

nothing: 12
Gold:6
Rubies: 4
Both: 2

=24 (I hope)

LordEmsworth Wed 30-Oct-13 10:18:45

well - if 12 found nothing, then 12 found something

14 items were found by 12 pirates - so 2 pirates must have found 2 items each

2 therefore found gold and rubies, 10 found either gold or rubies, plus 12 who found nothing ... equals 24 pirates

Wed 30-Oct-13 10:18:51

x-post

AliceinSlumberland Wed 30-Oct-13 10:18:54

Hahahah too slow!

Wed 30-Oct-13 10:19:02

Do we think the answer is 2 then?

Seeline Wed 30-Oct-13 10:19:09

Ah - so I did get hte maths right As usual it was the logic at the end that stumped me

Wed 30-Oct-13 10:19:35

Of the 24, 1/2 found nothing, so you're looking at a 'pool' of 12 who found something. Of those, 8 found gold, and 6 found rubies. This means that 6 of the 12 didn't find rubies, but must have found gold (else they'd be in the 12 who found nothing). As 8 found gold, 2 of those must have found rubies too - so my answer would be 2

I could be wrong, of course!

LeMousquetaireAnonyme Wed 30-Oct-13 10:19:50

8 found gold, 6 found rubies 12 found nothing, so 12 found something...

4 found only rubies, 6 only gold, 2 both answer is 2 found gold AND rubies

Wed 30-Oct-13 10:20:13

I think the answer is 2.

8 found gold, 6 found rubies, 12 found nothing.

So 12 found something, of which 8 found gold, so 4 only found rubies.
But we know that 6 found rubies, therefore 6-4 = 2 found both.

BoreOfWhabylon Wed 30-Oct-13 10:20:24

Surely it must be 2 who found both gold AND rubies?

Half (12) didn't find either

So the other 12 found one or the other or both.

BUT 8 + 6 = 14, not 12

So 2 must have found both. I think

Sparklebum Wed 30-Oct-13 10:20:49

I would say 2 found gold and.rubies

1/3 gold = 8
1/4 rubies = 6

so 2 found both rubies and gold???

FunkyFucker Wed 30-Oct-13 10:21:13

1/3 plus 1/4 plus 1/2 = more than 1... so add those together and the proportion over 1 found both.

BoreOfWhabylon Wed 30-Oct-13 10:21:39

cross posted with everyone else

Sparklebum Wed 30-Oct-13 10:21:54

Gah took me far to long to.type.that

Wed 30-Oct-13 10:22:52

Thanks everyone.

Wed 30-Oct-13 10:25:40

12 found nothing
6 found rubies
8 found gold

So 2 found both.

Failed maths gcse and had to do retake though!

larrygrylls Wed 30-Oct-13 10:28:32

It is that old topic: Venn diagrams.

12 found nothing, 8 found gold and 6 found rubies.

So draw two intersecting circles. The total number in the Ruby circle is 6, the total number in the gold circle is 8. The only way you can get to 12 finding something is 2 in the intersect, 6 in gold only and 4 in rubies only. The intersect represents finding both.

Someone above showed how to do it with equations, but Venn diagrams are nice and quick.

Try it!

SoupDragon Wed 30-Oct-13 10:35:29

I suppose the question should be worded "How many found both gold and rubies"

BerstieSpotts Wed 30-Oct-13 10:37:34

I read the question as how many found both, but I think Soup is right, to eliminate confusion it should have been worded like that.

BerstieSpotts Wed 30-Oct-13 10:38:21

I would find it very hard to calculate or visualise using venn diagrams. I think I'd have worked it out using the counting method above where it adds up to 26 meaning 2 must have been in two categories.

larrygrylls Wed 30-Oct-13 10:39:39

Umm, if the answer was 14, the question would have been worded "gold OR rubies". The word "and" implies both.

larrygrylls Wed 30-Oct-13 10:42:01

"I would find it very hard to calculate or visualise using venn diagrams."

I guess everyone finds different methods easier. However, I suggest that if you lay two big loops (hula hoops or something) on the floor with an overlapping area and you get two different kinds of objects, after a while they become pretty intuitive and easy.

Viviennemary Wed 30-Oct-13 10:42:44

I'll have a go. 12 didn't find anything. So that leaves 12

8 found gold
6 found rubies

So 14 found something. So how if 12 found nothing. Maths was never my strong point.

Wed 30-Oct-13 10:47:51

it's a logic question.

12 found nothing - leaving 12 others.

8 of those found gold - leaving 4 others. Those 4 others are included in the 6 who found rubies - so 2 must have found both gold and rubies.

OR

6 of those found rubies - leaving 6 others. Those 6 others are included in the 8 who found gold - so 2 must have found both gold and rubies.

Whichever way you do it - you come up with 2 who found both.

larrygrylls Wed 30-Oct-13 11:03:09

It is set theory, albeit v simple set theory.

BerstieSpotts Wed 30-Oct-13 22:12:00

I do know how venn diagrams work. It's just using them for this seems backwards to me. If there are 8 in the "gold" circle and 6 in the "rubies" circle, that tells you nothing about how many are in the overlapping circle.

larrygrylls Thu 31-Oct-13 07:34:59

Bertie,

If you know the total of rubies only, gold only and rubies and gold is 12, and you know the individual totals. You know that x + middle=6, y + middle=8 and x+y+ middle= 12. That is 3 equations in 3 variables, with the only solution of middle=2. A Venn diagram is a visual representation that most people find easier than solving the equations analytically. This is a fact as children are (or at least were) able to access the set solution long before they could solve 3 simultaneous equations in 3 variables.

Imagine if I introduced a 3rd category (sapphires, for instance) and gave enough info to solve, so 7 equations in 7 variables. 3 overlapping circles is just a v easy visualisation.

BerstieSpotts Thu 31-Oct-13 07:42:39

Yes, ok. I see your point. If you know the total then you can try overlapping them by different amounts and counting to reach the total.

Still it seems long winded But then most things which are supposedly "easier" I seem to find more confusing, I think my brain is just wired to do it in a different way.

larrygrylls Thu 31-Oct-13 08:33:56

To be honest, I personally prefer lots of equations to pictures. However I have come to realise that I am in a tiny minority. Maybe you are in the same minority! The vast majority of people are visual, though.

Also, set theory does eventually become v analytical and symbolic ( most of which I have forgotten). Venn diagrams are the access point to this branch of maths.

Ihatespiders Thu 31-Oct-13 09:11:33

With some clearer wording, this is a great puzzle and I shall pinch it for my class!

Makes a change from my usual variations on teachers sharing Quality Street and dropping unsubtle hints that I always eat the purple ones .

SoupDragon Thu 31-Oct-13 09:30:47

Umm, if the answer was 14, the question would have been worded "gold OR rubies". The word "and" implies both

I agree, but the fact that some people misunderstood the wording shows that it wasn't phrased clearly and that is easily rectified.

Thu 31-Oct-13 09:43:39

Funny how often maths comes down to being able to understand the language in the question rather than being able to do the maths.

Seems to me that being good at English is fundamental to being successful in any subject in our current UK system. Makes it very difficult for those who are late starters with reading and lose confidence in their own abilities.

LeMousquetaireAnonyme Thu 31-Oct-13 10:00:49

throck Language is very important in math and the teacher would have emphasise the use of small words like OR, AND.... So for the student who is listening the question should be very clear. It seems that the OP's son got it, so no pb there.

There is a math language that you have to learn!

larrygrylls Thu 31-Oct-13 10:04:14

"Seems to me that being good at English is fundamental to being successful in any subject in our current UK system. Makes it very difficult for those who are late starters with reading and lose confidence in their own abilities"

I think that you need to first access maths through words, as that is the only path in. As you get increasing feel for number and algebra, there are less and less words. On the other hand, in this instance, the precise meaning of "and" and "or" (and later Boulean "nand" and "nor") are almost like symbols in probability and set theory, so just need to be learned, like any other new bits of maths.

Maths does get a lot of time and effort in schools now, and people who start late should get support to catch up. However, I am not sure how much it really happens. I suspect some of it comes down to what parents do very early with respect to counting, sorting objects, asking "mathematical" type questions etc. I can't prove that, though, and I am sure others are far better able to comment on it.

Thu 31-Oct-13 10:14:01

I agree there are specific meanings for things in maths. But it seems there is very much a tendency for primary school (at least - don't know much about secondary level) maths and science to be very wordy. I'm not saying there shouldn't be some wordy questions - but the balance seems to be tipped too far that way IMO.

Not all people think in words - some people think visually or just in ideas.

You can easily test maths (even in a written exam) without making it a word puzzle which you have to get through before you get to the actual maths.

Although in my opinion the original question was clear and not too wordy. It clearly asks how many found gold and rubies, not how many found gold and how many found rubies (which is not far off the information you were given to start with).

Thu 31-Oct-13 10:26:08

Ds is delighted that there are 55 replies to his maths problem!!

Thu 31-Oct-13 10:31:38

Actually, I think the question was deliberately worded as it is. It's to make you think about the question you are being asked. Almost everyone will do the calculation as you did at first, and you then need to realise that, mathematically, your answer cannot be correct. My daughter, for example, would happily have answered '14' without thinking through to the next step.

SO, if your answer is incorrect, you need to review your interpretation of the question and work out what you are actually being asked. If the question was 'How many pirates found both gold and rubies?', you are spoon-feeding the examinee rather than asking them to really think about what they are being asked.

Mind you, it's a bit harsh if this is for 8-yos

DismemberedDwerf Thu 31-Oct-13 10:41:18

Blimey, after reading this, doesn't the word "found" look really odd?

Thu 31-Oct-13 10:43:59

Blimey - what year is your DS in?? That's completely flumoxed me!

Tue 05-Nov-13 08:24:33

I do know how venn diagrams work. It's just using them for this seems backwards to me. If there are 8 in the "gold" circle and 6 in the "rubies" circle, that tells you nothing about how many are in the overlapping circle.

Yes it does, your total has to be 12, but 8+6 = 14 so you know 2 must be in the overlapping section.

BerstieSpotts Tue 05-Nov-13 17:05:51

Clearly my knowledge of venn diagrams is lacking I remember covering them in year 5 when we were doing surveys and how to display results. I have never used them to work things out like said here.

reddidi Wed 15-Jan-14 01:59:32

Just to confirm, the use of Venn diagrams to solve this kind of problem is in the Key Stage 2 syllabus.

Also, translating word problems into mathematical concepts is a key skill in Key Stage 2 (and beyond).

IMHO this is a good question - and as he got the right answer, your DS would probably agree!

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