# 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.......

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

Message withdrawn at poster's request.

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.

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 .

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