Help me work out my sons maths challenges please

(22 Posts)
notnagging Tue 20-Nov-12 18:27:49

My ds loves maths but I am no good at it. He set me the following that took me ages to work out.
There are 2 brothers & 2 sisters. The years between the boys is 2, the same as the girls. The youngest boy is double the age of the oldest girl. There ages add up to 42 altogether. How old is the youngest child?

How did you work it out & how long did it take you?

noblegiraffe Tue 20-Nov-12 18:37:53

I called the youngest boy b and the youngest girl s
Eldest boy is b+2
Eldest girl is s + 2

Total of ages is b+b+2+s+s+2 = 42
So 2b+2s=38

Youngest boy is double eldest girl
So b = 2(s+2) = 2s+4
So 2b = 4s+8

Substitute into total age equation
4s + 8 + 2s = 38
So 6s = 30
So s=5

The others are all older than the youngest girl so that's your answer

janek Tue 20-Nov-12 18:38:04

Trial and error. About 5 mins, but was doing other stuff at he time...

mumsneedwine Tue 20-Nov-12 18:39:11

Youngest is 7. Girls are 7 & 9 and boys are 14 & 12. Took me about 3 minutes and worked it out as knew they must all around 10 (42 /4) and then used a bit of trial and error.

noblegiraffe Tue 20-Nov-12 18:39:15

Took me a couple of minutes but I'm a maths teacher wink

mumsneedwine Tue 20-Nov-12 18:40:21

And I think I read it wrong (how many times do I tell the kids to read the question!). I'll try again !!

mumsneedwine Tue 20-Nov-12 18:43:11

It's 5 !! So girls are 7 & 5 and boys are 16 & 14 (so youngest boy is double oldest girl). Quite enjoyed making brain work while cooking tea !

notnagging Tue 20-Nov-12 18:43:49

I don't know how a 9 year old could find this fun.

notnagging Tue 20-Nov-12 18:46:21

How long did it take you mumsneedswine?I've told him the answer so he's busy writing me another one

amothersplaceisinthewrong Tue 20-Nov-12 18:47:34

Oh, my son used to love all this sort of thing. Ended up with a Maths degree

Took me about 5 mins, worked out like the first poster

mumsneedwine Tue 20-Nov-12 18:53:06

Well once I'd read the question properly, it took a few minutes - but I do have a science degree and used to be a finance manager. Bit geeky and still love a bit of maths.

Niceweather Tue 20-Nov-12 21:15:22

Not even going to attempt!

CURIOUSMIND Tue 20-Nov-12 22:31:11

30 seconds!
youngest girl x, the oldest girl x+2, the youngest boy 2x+4, the oldest boy 2x+6.
x+(x+2)+(2x+4)+(2x+6)=42
x=5

notnagging Tue 20-Nov-12 22:34:53

He wants to be an accountant.smile My family is proof that intelligence is not inherited. 30 seconds. I won't be saying how long it took me!blush

noblegiraffe Tue 20-Nov-12 22:42:07

Go to YouTube and look up Dara O'Brian's School of Hard Sums. There are some nice puzzles on there, my secondary kids love the Chilli Chocolate game which is the second puzzle on episode 3. If you can understand the explanation of how to always win if you go first, you could beat him a few times and show him mum isn't that daft grin

notnagging Tue 20-Nov-12 22:43:58

Thank you noblegiraffe. Will dosmile

TuftyFinch Tue 20-Nov-12 22:45:56

16, 14, 7 and 5.

TuftyFinch Tue 20-Nov-12 22:52:16

Sorry I had to post that before it fell out of my head. It took about 4minutes in my head. I started at 12 for the youngest boy and was out by 2 just had to adjust by 2. I really enjoyed it but it hurt my head. I wanted to get a pen but struggled without. I'm quite pleased.
I don't know how to do it properly though.

notnagging Tue 20-Nov-12 23:26:10

I was like that tufty. Once I cracked it just had to get it out but took me a lot longer smile

TuftyFinch Tue 20-Nov-12 23:35:16

I keep looking at noblegiraffe and curiousmind's workings out but it's hurting my head! I do like maths problems <sadoddface> but can't do them 'properly'. DCs are still little but (4 and 5) but I feel I need to steel myself for all if this to come <confusedhurtheadface>

notnagging Wed 21-Nov-12 01:12:29

I know what you mean tufty. I can't help mine with hwksad

Niceweather Wed 21-Nov-12 06:33:04

Thanks for link Noblegiraffe. My son loves Dara so I will sit him in front of it.

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