coverOn 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.

23 comments

  1. 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. 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 😉

  2. 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”.

  3. 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. 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!

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