On Friday I posted this puzzle….

I saw a friend of mine the other day.  When I asked him how old he was, he said, “In two years I will be twice as old as I was five years ago.” How old is he?
If you haven’t tried to solve it, have a go now.  For everyone else the answer is after the break.
My friend is 12.  Any other solutions?
I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle (UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.

1. gavin Ledda says:

Hhahahaha i can’t answer it its just too good! 🙂

1. Ali says:

At first I got 8 because 8+2=10 and 5-10=5

2. Appocomaster says:

It’s fairly simple algebra: (x+2) = 2(x-5) – in 2 years my current age (x) will be equal to twice my age five years ago
this solves to:
x + 2 = 2x – 10
x -x + 2 + 10 = 2x – x -10 + 10
12 = x .

1. That was how I solved, afterwards I realized it can be simplified even more. In +2 year he will be twice the age he was in -5 years, so in 2+5 =7 years his age doubled, which means he was 7 5 years ago, and thus is 12 today.

I know I do the same thing as solving the equation, but this way I can do it all in my head.

2. Goliath says:

That’s what I did. Finally a fairly simple math problem.

3. @Sceptic_Tank says:

These were my workings, as taught for GCE O level, which dates me!
x+2 = 2(x-5) = 2x-10
add 10 to both sides: x+12 = 2x
deduct x from both sides 12 = x

It’s the same as yours, but I think it’s easier to follow if algebra is not a strong point 😉

3. Pitmonster says:

Got it in less than a minute, with some algebra. Nice one.

4. Baker's Dozen says:

You have a 12 year old friend ?

5. Anonymous says:

I didn’t use algebra…I used me noggin!

6. Natalia says:

Your 12 y.o friend will have great future if he is already likes so much math :)and puts tricky questions:)

7. Police Bureau says:

To be honest, this kid sounds like a bit of a dick. Why couldn’t he have just said “12”? Next time someone asks me how old I am I’ll say “The units are twice the tens and the total is the square of the units”.

1. Eddie says:

36

8. Eddie says:

Well done.

9. Dharmaruci says:

but what if he was born in BC? so the math is negative

10. issahamati says:

thanks

11. One Eyed Jack says:

Very disappointing puzzle. Simple algebra that can be done in your head makes me yawn.

12. I made a rookie dyslexic mistake early on in my mental arithmetic, adding the two at some point ending with 14 as an answer. What a numpty!

13. Fotoflex says:

I’m unfamiliar with algebra, but I think it would help when you can use it to solve problems with bigger numbers, just “fill in the blanks”, so to speak.
I could solve this one easily, but not if the question were to be ; ‘ in 3,456 years I’ll be 1.45 times as old as I was 576 years ago.

1. Hugh Jarse says:

9,536

2. ChrisR says:

Very perceptive.
Often people put up mental blocks thinking they can’t ‘do maths’ when actually there’s a lot of common sense.
In this case, you don’t know the person’s age so you make that an ‘unknown’ and call it ‘x’. It’s then a matter of rewriting the English of the question to get (x + 3456) = 1.45(x-576)
I agree that sometimes things can get complicated (working out 1.45 times 576 may need a calculator – and to check that the answer you get is like one and a half times six hundred) but if you take it a step at a time, aim to get ‘x’ (or a number of ‘x’s) on just one side and remember to treat each side of the ‘=’ the same (imagine they are your children!) – you’ll get the answer that you are currently 12,992 years old!
You are Dr Who and I claim my £5!

14. ChrisR says:

Gah! Hoisted by my own petard! yes, 9536

15. Jhow says:

seu amigo é bem jovem pra ser tão esperto!
your friend is too young to be so smart!

16. John says:

How do you work it out