Answer to the Friday Puzzle….

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On Friday I posted this puzzle….

A man spends a fifth of what is in his wallet, and then one fifth of what remained. He has spent a total of £72. What was the amount originally in his wallet?

If you have not tried to solve it, have a go now.  For everyone else the answer is after the break.

The man originally had £200 in his wallet! Did you solve it?

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.

32 comments on “Answer to the Friday Puzzle….

  1. kroketje says:

    yep🙂

  2. Anonymous says:

    too easy…………

  3. Julie C says:

    Nice bit of maths

  4. David C says:

    Looks a nice piece of thinking — except the answer surely is £112.50

  5. David C says:

    Sorry thinking about the wrong part of the question. Go back to school

  6. VicinCA says:

    Yes.
    100% – 20% = 80%
    80% of 80% = 64%
    Then: 100% – 64% = 36% (the amount remaining in the wallet £72)
    So £72 is 36% of what the original amount was.
    Therefore there was £200 in the wallet originally.

  7. VicinCA says:

    oops, not the amount remaining in the wallet, rather the amount spent out of the wallet. There’s actually £128 still in the wallet.

  8. Jeffrey says:

    Whatever happened to good ol’ algebra?
    O.2x + (0.8x * 0.2)=72
    0.2x + 0.16x = 72
    0.36x = 72
    X = 200

  9. -M- says:

    Yes, got it. Liked the puzzle, because took some time to get to the answer, and there was only one answer. It was also solvable without language problems.

  10. Anders says:

    Cool to see people thinking in %, It seems much simpler/ obvious now, but I did it with algebra:

    (X/5)+((X4/5)/5)=72
    (X+X4/5)/5=72
    Multiply both sides with 5
    X5/5+X4/5=X9/5=360
    X=360/(9/5)=360/9*5=40*5=200
    X=200

  11. lindamp says:

    I did it with algebra too, but not quite the same way.
    1/5 + (1/5 * 4/5) = £72
    1/5 + 4/25 = 5/25 + 4/25 = 9/25 = £72
    So 1/25 = £72/9 = £8
    So 25/25 = £8 * 25 = £200

  12. Jeremy says:

    You can do it in your head without all these math equations.
    Start with 25/25ths – remove a fifth = 20/25 remaining, remove a fifth, that is four more. So we have taken 9/25ths which is = 72. If 9/25 x X = 72 then x = 72/9 x 25 = 8 x 25

  13. Henry says:

    I used algebra too, and as soon as I saw the answer I realized that I should have intuitively known it by noticing that 72 is the sum of 40 and 32. My 9-year-old son saw it that way, and got it instantly.

  14. VrrChurr says:

    I also came to the conclusion that he originally had 112,50.
    For 72,- are 4/5 of what he had in his wallet in step one:
    72:4 = 16; 16*5 =90
    So before spending 1/5 he had 90,-
    Again 90,- are 4/5 of what he originally had.
    90:4= 22,5; 22,5*5 =112,50.

    Let’s assume he had 112,50 and spends 1/5
    112:50/5 = 22,5
    112:50-22,5=90
    Now he still has 90,- and again spends 1/5
    90/5 = 18
    90-18 = 72
    QED

    Now lets assume he had 200,- and spends 1/5
    200:5 = 40
    200-40 = 160
    So now he has 160 ,-
    If he again Spends 1/5 he still has
    160:5 = 32
    160-32 = 128,- left in his wallet.
    128≠72
    Therefore the answer 200,- is incorrect.

  15. Harpunakis says:

    My calculations show that it is also possible the answer to be 300 !!
    See: X/5+X/(5×5)=72 >> 6X/25=72 >> X=(72×25)/6 >> X=300 !!
    And to prove it: 300/5=60 .. and 60/5=12 .. so 12+60=72 !!

  16. VicinCA says:

    Your second fraction is incorrect. It should be 4X/(5 • 5) since only 4/5 of the original amount remains in the wallet before spending the second 1/5.

    X/5 + 4X/(5 • 5) = 72
    5X/25 + 4X/25 = 72
    9X/25 = 72
    9X = 1800
    X = 200

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  21. Rose says:

    Can anyone tell me if this is appropriate to be given to a Year 5 kid?

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