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

[Spidermama]

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?

Maybe they are assessing the children - perhaps they want to see if anyone is capable of doing it?
My problem with that would have been if the teacher hadn't let it known whether it had been taught in class how to solve this, and if so, how it was taught. I worry that I am teaching my dd the wrong problem solving methods because I teach her the way I do it (inevitably the wrong way, I am not good at maths).

Cat1+(2xCat2s) +Cat3 = 15
Cat1 +Cat3 = 15 - (2cat2s)
11 = 15 - (2cat2s)
2cat2s = 4
Cat2 = 2
And with that you can easily find out that cat1 is 5 and cat3 is 6

I think it's meant to be a trial and error thing, not calculated as such... like if the first one is 1kg and 2 is 6kg, then 3 would need to be 10kg, which doesn't work. Etc.

Hard, but not waaaay too hard (with a DD in Yr3 myself). Would need my help to think about it, and have confidence to try some answers, rather than "knowing" answer straight away.

What type of school is this by the way? Seems very complex for a 7yr old

I think ds1 had stuff like this in year 4. We had to phone his Dad in the end...

its simultaneous equations and when I was at school this was taught in secondary. Yes, its amazingly difficult for a child of 7!

The answer is 5, 2 and 6 (as has been noted up thread) but I have a degree in maths and needed to write down a couple of lines of working out before I could solve it.

Is this G&T maths maybe?

I wonder whether maybe, because we know how to do simultaneous equations, we would do it that way without realising that it is simpler than it looks. It is actually a logic problem which needs neither G & T maths, nor trial and error.

From the first 2 statements, we can easily work out that Cat3 weighs 1 kg more than Cat1.

Then from statement 3, we can work out that there is only one possible answer and that Cats 1 and 3 must weigh 5 kg and 6 kg. We can then use either of the first 2 statements to work out what Cat2 weighs.

Then again, none of us are 7 year olds.

That's how I did it CecilyP - and i think dd1 (7) would get there that way too if she thought about it.

I gave it to DS to do this morning. He wrote done the answer almost immediately. He is quite good at maths though (just turned 8).

He sauys can he have more please. I think he especially likes the fact that it was about cats.

I'm feeling quite chuffed: DD (age 8) got the right answer in about 5 minutes using the plausible trial and error method as I'd hoped. DS2 (age 11) took one look at the question, scribbled the right answer straight away, and when I asked how said 'I could just see it'. I asked him about the Rattitude's method and he said "yeah, yeah, I could have done it like that".

Pythagoras' theorem
Poisson distribution
Rattitude method

I worked it out using simple algebra, but I'm not sure how I'd have got there aged 7. Trial and error, probably.

I've just showed this to my DD who as just started Year 4. We did it by prediction, the 1st cat could only weigh 1 - 6 kg and took it from there.

She got a 4c in Maths in the optional end of Year 3 SATS, so I think she's doing ok in Maths. She needed support in solving this one though.

It's a bit of logic too. Which doesn't hurt

"I just googled Mathematical Puzzles for the more Able Kingscote and couldn't find it on Amazon - is it a book that parents can use or just one for teachers? "

It was a Government publication, not a commercially available book.

Full version available on the web here:
www.bgfl.org/bgfl/custom/files_uploaded/uploaded_resources/12212/mathspuzzlesall.pdf

I use them a lot in teaching. It's not always the best 'calculators' who are the best puzzle solvers (I have seen my 'most able' in tears over a puzzle that my least able were solving easily with a practical method involving counters or dots), partly because many children who are seen as able at maths can't cope with the idea of there being potentially more than 1 right answer, or are not logical in their approach.

3rd must be 1kg more than 1st so 1st and 3rd are 5 and 6 respectively. That leaves 2nd cat at 2kg. Tricky for a 7yo but not impossible.

Maybe he is very bright and the teacher is challenging him as far as they can.

I showed it to my Y3 child- she curled up into a ball and started crying I have completely forgotten algebra and even looking at the "explanations" here I get a kind of word blindness.

Thankyou teacher I will have a look. I like the idea of giving dd interesting msths puzzles - I don't get much info about what she's doing at the mo do am not surewhst to do at home.

I like Raattitude's method - neat...

add them all up 7+8+11 =26

you have 2 of each cat so divide by 2 =13,
cat1+cat2=7
so cat3 =6 and off you go...

probably works for all combinations of cats!

It's from a maths pack designed for Y3 and 4, the idea is to select an appropriate level of challenge for the child.
So he needs to have the strategies required to solve the problems already in his repertoire.

Solve mathematical problems or puzzles.
Know addition and subtraction facts to 20.
Explain methods and reasoning.

If he hasn't done a lot of reasoning, talking and playing around with problem-solving in Y2, the challenge is very high.

I taught year three for a number of years and I would teach them to tackle that as a 'trial and error' problem, where they try different answers until they get the right one. It's a legitimate method for solving problems. Hope that helps. Make sure they show the workings, the process they use is probably more valid than getting to the right answer...

Starting with titty's a,b and c

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

rearange and substitute

a+8-c=7
a-c=-1
a+c=11

a=-1+c and a= 11-c add these two to get 2a=10 so a=5

now you know a=5 you can put that in the other occasions

5+b=7 so b=2

c=11-5 sp c=6

There is a similar problem in one of DD's Brain Training DS games and I have taught her to do it by trial and error which she grasped quite readily. She has just gone into Yr3

I believe they do this by trial and error - that is a learning method too. So we start with say a=4 - does it work, yes - continue, no - change a etc. They use logic and learn about scenarios :)

Gave it to my dd, 7, just started year 3, and she worked it out quite quickly but she is a maths whizz and so not very representative of seven year olds as a whole.