# Talk

## Is it me or is this a RIDICULOUSLY hard maths problem for a 7 year old?

(82 Posts)
Spidermama Thu 20-Sep-12 16:21:34

New school. He's in year three and this is the homework problem he has been given.

Keiron has three cats. Each is a different weight.
The 1st and 2nd weigh 7kg together.
The 2nd and 3rd weigh 8kg together.
The 1st and 3rd weigh 11kg together.

What is the weight of each cat?

<I've worked out the answer just by using several guesses until I got it right but I'm assuming there's a more logical method I can help DS with?>

Any ideas? It's due in tomorrow.

SuePurblybilt Thu 20-Sep-12 16:26:29

Jakers. I'm fecked when DD gets to Year Two then.

I suppose you start with the first statement and work out the only weights they could be. Then the same with the other two and sort of eliminate.

You're right, there must be a simple way.

starfishmummy Thu 20-Sep-12 16:27:39

Eek!

smalltown Thu 20-Sep-12 16:28:21

Message withdrawn at poster's request.

smalltown Thu 20-Sep-12 16:29:41

Message withdrawn at poster's request.

scaevola Thu 20-Sep-12 16:30:06

I approached it as a refining guesses exercise. There can be no more than 6 possibilities (as A+B=7). So I made a table giving all the possible values 1-6 for A, which in turn gives all possible values for B. Then you use B to give all possible values for C, then by adding A+C according to the table, you find the only possible answer.

Frontpaw Thu 20-Sep-12 16:30:17

Maybe you have to make an assumption with one of the statements

Is it 5, 2 and six? I need some paper...

RustyBear Thu 20-Sep-12 16:30:45

2nd and 3rd weigh 1kg more than the 2nd and 1st, so 3rd weighs 1 kg more than the first.
So then, they have to find two numbers that add up to 11, where one is 1 more than the other ie 6 and 5. So 3rd is 6kg, 1st is 5kg, so 2nd must be 2kg

tittytittyhanghang Thu 20-Sep-12 16:31:03

Is this a bit like introducing algebra.

a+b=7
b+c=8
a+c=11

cant be bothered to think where it goes from here dont know

Frontpaw Thu 20-Sep-12 16:32:06

Sadly I can work things like this out but have no ideas how I do it. It's a bit like Little Man Tate on a much simpler basis. My mum taught me, and she didn't know either.

Spidermama Thu 20-Sep-12 16:33:06

The answer is here (no 39) but there's not indication how it was worked out.
We are supposed to show workings. Also its not much of a learning exercise if we just guess is it?

Frontpaw Thu 20-Sep-12 16:33:12

And I hate these... Sally is 2cm taller than the person on the right, who is 3cm shorter than the person to her left... I get bored with these.

LilyBolero Thu 20-Sep-12 16:34:23

First instinct is to do it with simultaneous equations, but tbh a 7 yo will just do it with number bonds and trial and error. So they could say that they know 6+5 is 11, then try the other two sums with 6 + 5 as 1 and 3. Using 6 as 1 doesn't work, so use 5 as 1, 6 as 3, so 2 has to be 2, and that works.

Ladymuck Thu 20-Sep-12 16:34:51

I think that for year 3, they are looking for trial and error, and for someone not to be daunted when the answer isn't obvious.

Otherwise:
If Cat1 + cat2 = 7 and cat1 + cat3 =11, then you know that cat 3 is cat2 + 4. Using the 2nd statement this means that cat2 + cat3= Cat2 + cat2 + 4=8, so cat 2=2. Cat3=cat2+4=6, and you can then work out that cat1=5.

Merrow Thu 20-Sep-12 16:34:58

Yeah, I would have done it with algebra:
x+y=7 therefore x=7-y
y+z=8 therefore z=8-y
x+z=11 therefore 7-y + 8-y = 11, 15+2y=11, 2y=4 y=2, x=5 and z=6

No idea how I would have done it at seven!

Spidermama Thu 20-Sep-12 16:35:28

Well done smalltown.

My brain's hurting. There may be trouble ahead.

BillyBollyBandy Thu 20-Sep-12 16:35:41

I worked it out the same way as rusty

GoldPlatedNineDoors Thu 20-Sep-12 16:36:45

I would have done it with algebra too. Am astounded at use of algebra at that age though

crikeybadger Thu 20-Sep-12 16:37:08

Yes, I'd say that is tricky for a 38 year old 7 year old.

My DS is in year 3 and his homework consisted of understanding the concept of hundreds, tens and units.

Bookbrain Thu 20-Sep-12 16:37:47

If you look at the first and second statements, you can see that the 3rd cat weighs 1kg more than the 1st cat.

With than information, it should be relatively easy to look at the third statment and work out how much each cat weighs. And from there work out how much the 2nd cat weighs.

It does seem awfully complicated for a 7 year old though. Maybe fun and interesting if they are advanced in Maths and ready for some logic/algebra type homework. Is this extension stuff for G&T-type children or what everyone has been given?

OlympickingMyNose Thu 20-Sep-12 16:37:52

1st weighs 5kg
2nd weighs 2kg
3rd weighs 6kg

But geez! For a year 3! Yes, too hard!

lljkk Thu 20-Sep-12 16:40:25

It's simple algebra, but I would expect a problem like that for y5-7, not y3.

witchwithallthetrimmings Thu 20-Sep-12 16:41:19

I reckon they are teaching the skill of showing that something CANnot be the case
so if cat1 = 1, then this means cat2=6 and cat3=10 but cat2+cat3=8 which is not 6+10
so lets try cat1=2 and so on

Rattitude Thu 20-Sep-12 16:42:07

The 1st and 2nd weigh 7kg together.
The 2nd and 3rd weigh 8kg together.
The 1st and 3rd weigh 11kg together.

If you sum up the three statements, you get 7+8+11= 26kg, which is double the sum of the three cats.

So the sum of the three cats is 13kg.

Based on that and the first statement, you know that cat no. 3 weighs 6kg and you can derive the weight of the other 2 cats from the other two statements.

Jinsei Thu 20-Sep-12 16:44:41

What rusty said. My 7yo dd would love homework like this.

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