Maths Reasoning question....help please

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SleepyHead
Posts: 484
Joined: Thu Oct 18, 2012 10:41 am

Maths Reasoning question....help please

Post by SleepyHead »

Ben's father was 33 when Ben was born. 8 years ago Ben's father was twice as old as Ben is now. How old is Ben?

Any answers greatly appreciated

SleepyHead
Bestichka
Posts: 63
Joined: Tue Jan 17, 2012 5:29 pm

Re: Maths Reasoning question....help please

Post by Bestichka »

Say, Ben is B and Father is F.
Say, B = x, and then F = (x+33).

Eight years ago it was like this:
(X+33) - 8 = 2x
X +33 - 8 = 2x
X + 25 = 2x
25 = 2x-x
25 = x

So Ben is now 25, and his Father is now 25+ 33 = 58. Eight years ago Father was 50, which is 25 x 2.

(No doubt, Father got lots of grey hair after pushing his DS Ben through selective exams ;) )
SleepyHead
Posts: 484
Joined: Thu Oct 18, 2012 10:41 am

Re: Maths Reasoning question....help please

Post by SleepyHead »

Excellent thanks, will explain to DD, who now thinks mummy can't do the 11+ :oops:
Guest55
Posts: 16254
Joined: Mon Feb 12, 2007 2:21 pm

Re: Maths Reasoning question....help please

Post by Guest55 »

Use logic rather than algebra - algebra is not necessary!
yoyo123
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Location: East Kent

Re: Maths Reasoning question....help please

Post by yoyo123 »

diagrams help a lot too
JRM
Posts: 301
Joined: Sun Sep 09, 2012 7:32 pm

Re: Maths Reasoning question....help please

Post by JRM »

Guest55 wrote:Use logic rather than algebra - algebra is not necessary!
Can you explain how you would do this using logic rather than algebra? I think in algebra - although obviously less formally - as does DS1. But DD is different and I wouldn't really know how else to explain it well to her.

Thanks.
The more that you read, the more things you will know.
The more that you learn, the more places you'll go.
Dr Seuss
Guest55
Posts: 16254
Joined: Mon Feb 12, 2007 2:21 pm

Re: Maths Reasoning question....help please

Post by Guest55 »

Hi

Well I said the difference in their ages is 33 - but we are looking at one of them 8 years ago.

33 - 8 = 25

So effectively a difference of 25 - so that's Ben's age and his Dad then was 50.
JRM
Posts: 301
Joined: Sun Sep 09, 2012 7:32 pm

Re: Maths Reasoning question....help please

Post by JRM »

Thank you so much. That still took me ages to think through. I could see that you were right, but it still took me time to convince myself that there were no other options going that route. I think I have just proved the argument for why people need different methods to see which suits them best. If DD finds this sort of reasoning easier than algebra then I shall be back for some more help in a couple of years. It should be so obvious - maybe I am just tired? (Desperately trying to think of excuses for myself).
The more that you read, the more things you will know.
The more that you learn, the more places you'll go.
Dr Seuss
Guest55
Posts: 16254
Joined: Mon Feb 12, 2007 2:21 pm

Re: Maths Reasoning question....help please

Post by Guest55 »

Age problems are always tricky - the algebra method would have been easier to follow if Ben's age = b so dad = b+ 33.

Using 'x' for the unknown just makes it harder! Always use letters that link with what they represent ..
Bestichka
Posts: 63
Joined: Tue Jan 17, 2012 5:29 pm

Re: Maths Reasoning question....help please

Post by Bestichka »

Sorry, Guest55, I was using letters the way i did above as i was educated abroad and that was the way we were taught algebra - we were always using x for unknown. Hope no mistake there, just a matter of custom and convenience.
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