I have been given a guide on how to reach satisfaction, and part of this guide requires me to meditate exactly for 90 minutes.

I have only been provided 2 oil-soaked ropes which last for 2 hours each when lit, and matches. (To time myself with)

I don't have a knife on me, and I am too weak to break it with my hands.

How can I meditate for exactly 90 minutes?

(Note: If you have seen a similar variation of this please don't answer)

  • 2
    $\begingroup$ I have seen a similar (indeed more general) version of this, and it was on this site. VTC as duplicate (but also +1 since it's a good puzzle). $\endgroup$ May 15, 2015 at 21:54
  • $\begingroup$ I will come downvote any answer that requires you to 'lite the second rope' thus breaking the meditate :D $\endgroup$
    – Alex
    May 15, 2015 at 21:58
  • 2
    $\begingroup$ Wait, your guide says 90 minutes, but you want to meditate for 45? $\endgroup$ May 15, 2015 at 22:13
  • $\begingroup$ @randal'thor I should post this comment on Kendal Frey's question or diabonas' answer, but both appear to not be active on this site (last post in 2014); I hope discussing that answer here is ok but if not, I'll post this as a new question. The answer by diabonas is interesting, but only refers directly to the $\frac{m}{2^n}$ form in the sum near the bottom, and then only (it seems) relies on a common $2^n$ denominator for $t_{r,l}$. If we changed the form from the start to have a $3^n$ denominator, say, or indeed any other denominator, the argument still seems to hold. What am I missing? $\endgroup$
    – Lawrence
    May 15, 2015 at 22:38
  • $\begingroup$ Sorry I meant 90 minutes, I just forgot to change the 45 because I was changing the question so someone else couldn't google it. $\endgroup$
    – CyanogenCX
    May 15, 2015 at 23:44

1 Answer 1


To time 90 minutes as your guide suggests:

Lay the ropes next to each other.

Light one rope from both ends.

When it burns out (1 hour)

light the other rope at the middle (this is why you lay them next to each other) and at both its ends.

This second rope will burn for

30 minutes

, giving you a total of 90 minutes.

  • $\begingroup$ question asks for 45, but guide says 90 I think that was a typo. $\endgroup$ May 15, 2015 at 22:12

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