It’s the Friday Puzzle!

77

Please do NOT post your answer, but do say if you think you have solved the puzzle and how long it took. Solution on Monday.

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?

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.

77 comments on “It’s the Friday Puzzle!”

1. tweetyaca says:

20 seconds. No calculator needed.

2. Amy says:

Took me about a minute. A satisfying answer which I know is right – I’ve struggled in past few weeks!

3. Niva says:

A straight arithmetic problem with easy numbers no less. <30secs.

4. kroketje says:

Easy one, less than 1 minute

5. Al says:

Yep – few minutes

I’m not as clever as the other dudes who post on this site so it has taken me many hours to find an answer that I am still not fully confident with.

• Skyfire says:

Your honesty made my day – I often feel the same with the most puzzles 😉 !

• Anonymous says:

one fifth of one fifth is one twenty-fifth. one twenty-fifth plus one fifth is six twenty-fifths. seventy-two equals six twenty-fifths. six multiplied by four equals twenty-four. seventy-two multiplied by four equals two-hundred eighty eight. seventy-two divided by six equals twelve. two-hundred eighty eight plus twelve equals three hundred dollars. thirty seconds, no calculator, I am eight years old.

• Anonymous …
1) don’t be an idiot and post the answer, it clearly says not to
2) don’t be a double idiot by posting the wrong answer

7. Sean says:

About 2 minutes no calculator just good ole fashioned brain power 🙂

8. Selim says:

it’s ok

Yeah, less than a minute I think, including a little chat with myself to have a go. Used a calculator though…

10. Steve Jones says:

Something of a mugger’s wet dream given the amount of cash he was carrying.

• Good point. Other than professional (or amateur) gamblers, who would carry around THAT much cash with them?

11. Julie C says:

I got caught out at first my misreading the question but I’m happy with the answer.

12. Appalie says:

I seem to have found the answer in less than 2 minutes… However, is there catch?

13. Tony says:

Around 1 minute – got a surprise with the simple figures though !

14. Doug says:

You Brits are fast! it took me 5 minutes with a calculator and just plugged and chugged until I get it. American math system blows. Looking forward to the algebraic solution

• Anonymous says:

I’m surprised. I haven’t heard about plugging and chugging for a while. very reliable, though.

16. nulle says:

<30 sec in my head after reading the problem. the wording is confusing…took me longer to read the darn problem than solving it…

17. Harish says:

straight math, easy numbers.. looks like this was for an ego boost. Bracing myself for next week.

• Anonymous says:

Harish P? I know a Harish at my school.

• Harish Jayagopal says:

Am Harish Jayagopal, that must b someone else.

18. Ron says:

5 sec of real thinking.

19. Anonymous says:

Liked the puzzle. I wish I could write puzzles like this.

20. Captain Wow says:

Hazarded a guess, and the maths showed I got it right first time.

21. VicinCA says:

Nice one. About a minute or so to solve.

22. drewbk says:

Easy peasy, but love the maths ones 🙂

23. Paul Durrant says:

24. It boils down to an equation. I’m surprised that people got it without using pen and paper.

25. Alistair says:

Played on a whiteboard, assuming £100 to start so the fractions would be easy. Then realised what the real answer would be. Now I just need to figure out how I could have done it with just brain power alone.

• Anne Elk says:

I can NEVER do these without a pen and paper! I don’t know where I’d even start if I only had a calculator…

• Mervulon says:

Working it in reverse order, with just a tiny bit of initial logic, and it’s quite simple to do in your head.

26. Sue Teean-Sweap says:

It’s a good one. Went wrong the first time and concluded that he had less money to begin with than he ended up with. Then I realised the error. Two minutes – with good old-fashioned algebra.

27. Stewart says:

About 5 minutes. Had to use a bit of algebra. I’m not as sharp as when I was was much younger.

28. Dave says:

Weird. Instinctively thought of an answer as I read it and it turned out to be correct.

Evidently I’ve spent too long thinking about higher rate pension contributions with the 20% grossing up followed by the 20% PTR rebate.

I bet you say that to all the girls, Dave – what an awesome chat up line.

29. Dave says:

Boring!

30. pete says:

wow – even a thicko like me got that in about 10 seconds 🙂

31. Natura Umana says:

5y=4x
x+y=72
5x = initial value;
i think this is it.

32. Dharmaruci says:

i am confused by phrase “and then one fifth of what remained” does this mean one fifth remained in his wallet? so he spend fourth fifths?

33. Matt says:

At first I thought this would be a tricky maths question that I’d need a calculator for.
Then I made an estimated guess in about 5 seconds, and rationalised that the first ‘one fifth’ would have to be exactly one fifth more than the second ‘one fifth’.
Subsequent calculations revealed my guess to be correct!

34. Lazy T says:

I ‘ready reckoned’ the answer in seconds then wasted about 10 mins filing the back of an envelope with gibberish.

35. Got it. 1 min of simple math. I hope I’m right.

36. JC says:

Just chugged in some numbers, until I got it nicely to 72

37. Fotoflex says:

I think I worked it out, I just picked a nice, round number to start with and found I only had to double it! That took about a minute.

38. Julia says:

For once I used Maths instead of trial and error. Took about 3 minutes, but then I’m not very good at Maths.

39. Calgacus says:

Several minutes using algebra, with my fuzzy “just woke up” head on.

40. Duncan says:

A few minutes using simple algebra, I’m still trying to think of a neater solution though.

41. Evan Cohrs says:

He would have spent £200 even. 200/ 5= 40 40- 200= 160 160/ 5= 32 40+ 32= 72 Thanks

– Evan Cohrs

• Anne Elk says:

Brilliant, genius. I’m sure you know how to tie your shoelaces as well, although I’m not sure I’d trust you to sit the rght way round on the toilet. Unfortunately you didn’t know how to read the bit about not giving the answer away.

• The other Matt says:

Same here. I tried it out with several values. I was sure thats a even number. But how is the correct formula ? What if the spent were 194654 Pounds ? Haha ?

42. mikekoz68 says:

This puzzle is lame, anyone with grade 9 algebra could solve it, I agree, the question takes longer to read than solve! Straight Algebra….where’s the puzzle?

43. Eddie says:

I’ve worked out that if he did this iteration 45 times he’d have less than a penny remaining.

44. Martian Man says:

Got it – initial attempt at solving the Friday quiz … about 1 minute

45. Anonymous says:

Got it, after reading all the hints and frigging with numbers for long enough. Still no clue what the formula should be. I probably shouldn’t be in university.

46. Anonymous says:

Took a minute to find a scrap of paper, a minute to write equations, then a minute to solve.

47. SofARMaths says:

Less seconds than the answer in £…

48. Phil H says:

49. Anonymous says:

50. Manos says:

That was an easy one… I spent a total of 72 seconds to solve it 🙂

51. mightyhero says:

Too easy.

52. Anonymous says:

I’ve given this to my math students at school. Just think clearly and write down what you think.

53. James Bailey says:

I seem to remember there is a proper method for figuring these things out, but not being able to remember it I resorted to heuristics. Managed to get it in a minute or so.

54. raaz says:

I took just too much time as compared to all the others 😦
first tried with equations and got wierd results, then with a random number and got the correct value (by chance) and then went back to the equations and got it,…
I am sure on Monday we ll see a different way of resolving it with much less paper and pencil….

55. doug says:

about 5 min, used my phone tho, as paper, could have done it faster if I had actual paper:) no calculator tho.

56. Fabio cunha says:

It took 5 minutes tops… Im pretty sure I have it right

57. Sari1967 says:

For the first time in ages I have got it quickly.

58. sushey says:

2 Minutes, inclusive finding something to draw on…

60. Louise says:

Ten seconds. 🙂

61. Mort says:

About 1 or 1-1/2 minutes using a calculator since one was right at my elbow. Probably could have just as easily done it using algebra and mental calculations.

62. I guessed what number it would be instantly. But wrote the algebra out and solved for x just for good measure 🙂

63. Ronda says:

I love algebra! But this was pretty easy–less than a minute. And I want to trade him wallets. 🙂

64. Teun Spaans says:

simple arithmetic, nice for a change. 10-15 secs

65. Quinton says:

I’ve been reading this blog for your excellent when. Keep up the brilliant job you are doing here.