You are given two candles of equal size, which can burn 1 hour each. You have to measure 90 minutes with these candles. (There is no scale or clock). You are also given a lighter.
OK. Old Infosys interview question. As @klm123 said, you burn both ends of one candle. It will burn in 30 minutes and you burn the other one from one end only.
Initially, the problem was with
ropes / threads with each of
timespan 30 minutes and you have to
measure 45 minutes with 2 ropes; with which it was quite affirmative and little easy. But someone (NOT the asker here) become thinking himself/herself very smart changed the problem with candles and increased time to double. The smarty who changed it probably had no / little understanding of Physics / Chemistry.The candle will burn quicker if burned on both ends by placing it horizontal (burn a little faster from both sides) or vertical (burn quite faster from one side). So, we won't get exact 30 mins but have to approximate.
The rope problem on math exchange and glassdoor . Take a look : https://math.stackexchange.com/questions/183282/puzzle-there-are-two-lengths-of-rope http://www.glassdoor.co.in/Interview/You-have-2-pieces-of-rope-each-of-which-burns-from-one-end-to-the-other-in-30-minutes-no-matter-which-end-is-lit-If-dif-QTN_108377.htm
If the candles burn at the same rate when sideways vs. upright (which they probably won't, but let's make the assumption for the sake of having a solution), lie the candles horizontally next to each other, but with the wicks at opposite ends. Light both. When they meet, they'll have both burned halfway - 30 minutes gone.
Now blow one out. Let the other burn the rest of its 30 minute lifespan - now you've timed 60 minutes.
When it goes out, relight the other one for the remaining 30 minutes - total of 90 minutes.
One way that assumes the candle are exactly the same is to use the unlit candle to determine when the first candle has burned half-way.
- Light the first candle and wait for it to burn to approximately half-way (must be before it reaches half-way).
- Place the unlit candle next to the lit candle and use your finger or thumb to mark where the lit candle is on the unlit candle.
- Invert the unlit candle keeping your finger/thumb on the position you measured in step 2 and compare it to the height of the lit candle again.
- Once the mark is the same on the first and inverted measurement you know the lit candle has burned exactly half-way. You can now light the unlit candle and wait for it to burn to the end.