Tricky question from Letts.. how to explain to year 4 child

[SleepyHead]

A farmer has three goats, A, B, and C. Each goat is a different weight and is an exact number of kilograms. Goats A and B weigh a total of 20kg. Goats B and C weigh a total of 15kg. Goats A and C weigh a total of 17 kg.

What is the weight of each goat?

Is there a simple way to do this?

Any help gratefully received
Sleepy

[secondtime]

Because of the last two statements, Goat A weighs 2kg more than B.

So what two numbers make 20 that are '2 apart'? Answer 9 & 11.

Therefore A is 11kgs and B 9kgs. It then follows easily C weighs 6kg.

[SleepyHead]

thank you secondtime, you make it look so easy.

[Okanagan]

Another simple way is to add 20kg + 15kg + 17kg = 52kg.
That will be double the total weight of all three goats as it is (A+B) + (B+C) + (A+C) - or 2 (A+B+C)
So the total of all 3 will be 26kg

We know A+B = 20kg - so C must be 6kg (i.e. 26kg-20kg)
We know B+C = 15kg - so A must be 11kg (i.e. 26kg-15kg)
We know A+C = 17kg - so B must be 9kg (i.e. 26kg-17kg)

[mystery]

Great question. Which book is this from? Does it contain others which are as good as this?

[SleepyHead]

Hi Mystery

I have Pmed you the link to the book

[SleepyHead]

Thank you Okanagan, that's a great method . Will explain both to DD

[SleepyHead]

Mystery,

There's another question in the same book (later on) so DD hasn't done this yet.. but here it is anyway

"Rosie bought three different hats. The red hat and the green hat cost a total of £16. The white hat and the green hat cost a total of £13. The white had and the red hat cost a total of £15. What is the cost of each hat?"

This appears on a level 4 paper too.

[somum]

Hi Mystery

I have Pmed you the link to the book

Hi SleepyHead,

I have Pmed you.
thanks

[bead99]

Hi SleepyHead,
Kindly PM me the link of the book.

Regards