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

(82 Posts)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.

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.

Eek!

Message withdrawn at poster's request.

Message withdrawn at poster's request.

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.

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

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

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

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~~

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.

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?

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.

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.

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.

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!

Well done smalltown.

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

I worked it out the same way as **rusty**

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

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.

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?

1st weighs 5kg

2nd weighs 2kg

3rd weighs 6kg

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

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

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

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.

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

Join the discussion

Registering is free, easy, and means you can join in the discussion, get discounts, win prizes and lots more.

Register nowAlready registered with Mumsnet? Log in to leave your comment or alternatively, sign in with Facebook or Google.

Please login first.