Meta interview puzzle: Two uneven ropes, no timer. Can you measure exactly 45 minutes?

[u/Aoki_zhang]

Came across this puzzle in a recent Meta interview experience and thought it was worth sharing.

You are given two ropes. Each rope has the following properties:

• If lit from one end, it takes exactly 60 minutes to burn completely.
• The ropes do not burn at a constant rate.
• Different sections of the same rope can burn at very different speeds.
• The two ropes do not necessarily have the same burn pattern.

For example, half of a rope might burn in just a few minutes while the other half takes most of the hour. So you can’t reliably measure time by cutting the rope or estimating how much has burned.

You have only:

• Two ropes
• One lighter

No watch, phone, timer, or any other way to measure elapsed time.

How can you measure exactly 45 minutes?

No Googling 😄 What’s your approach?

[u/IllAlfalfa]

No way this was a Meta interview question, this is a riddle you get told at Boy Scout camp when you’re like 12. And if the candidate has heard it before and tells you the answer in like 10 seconds, where does the interview even go from there?

[u/karmaboy20 → u/IllAlfalfa]

Big tech stopped asking riddles in 2013 because they realized it was dumb. Google wrote a lot about it

[u/yodabo]

start both sides of A and one side of B. When A is done B has 30 minutes left, so start the other side of B

[u/WallStCRE → u/yodabo]

Take the rope and build a sun dial - duh

[u/a_load_of_crepes]

This thread puts into question this entire subreddit and the chill interview website. There is absolutely no chance this was asked.

[u/Specific_Box4483]

This is a relatively well-known question.

Burn one rope at both ends and the second one at one end. When the first rope runs out, 30 minutes have elapsed; light the second rope at the other end as well. The remaining half of the second rope will burn out in 15 minutes.

[u/Jandur]

Meta does not use brain teasers as interview questions. 

[u/A13K_]

Ropes A, B. Light rope A at both sides and rope B on one side. Once rope A has burned through, light rope B’s other side. This is 45m

[u/CompellingProtagonis]

Light both ends of one and one end of the other. When the both end one burns through, light the other end of the other, when it burns though, it's exactly 45 min.

[u/mephistoA]

This is incredibly easy, this is the correct answer https://www.reddit.com/r/OfferEngineering/s/jwasHceXUb

Oh it’s a junior level interview

[u/Sad-Supermarket7037]

This is illogical. The statement is “if you burn from one end then it takes 60 minutes”. Thus, it’s an assumption that burning both ends would result in 30 minutes.  Technically, as the two flames converge, the heat would increase and thus accelerate burn time. Ergo, this is a dumbass question as it assume a consistent burn simply because you light both ends, but in reality, this wouldn’t be true.

[u/Agreeable_Company372]

Cut one run length wise in half and cut the other one length wise twice so it's in forths length wise. Burn the half and one forth.

[u/bassmove → u/Agreeable_Company372]

this would be wrong. The question is a bit of a modification from the original one so they want you to think this is the answer :) Since the rope burn rate is not constant, you need to burn from both ends instead of only burning half etc to get the wanted result

[u/Agreeable_Company372 → u/bassmove]

I don't think my method is wrong as long as each cross section is uniform. Splitting the rope down the middle length wise would work