# Candle timing measurement [duplicate]

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.

• So you can not light both ends of a candle to make it last 30 minutes? – klm123 Jul 26 '14 at 11:28
• Is it really calculation puzzle, or more physics puzzle? If yes, please reformulate it as abstractly as you can and tell what exactly is different here with this puzzle: puzzling.stackexchange.com/questions/166/… – klm123 Jul 26 '14 at 11:30
• This is a small piece of puzzling.stackexchange.com/questions/170/… – Ross Millikan Jul 28 '14 at 4:49
• P.S. I have not marked it as duplicate. I marked it as "Unclear what you asking". You can not judge here that this is duplicate. – klm123 Jul 30 '14 at 17:41

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.

• -1. A candle doesn't not burn with the same speed if the fair is not vertical and you can not make fair at both ends be vertical. – klm123 Jul 26 '14 at 11:52
• That's true. But you have to approximate. If we put physics in to the problem it will be really difficult/ impossible to find answer. One thing can be done. Put the candle horizontal and burn both ends. Nearly 30 mins. It was an interview question in a software company. – Tanmaya Meher Jul 26 '14 at 11:58
• @klm123 Although I believe this is the conventional answer to this puzzle. – arshajii Jul 26 '14 at 16:23

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.

1. Light the first candle and wait for it to burn to approximately half-way (must be before it reaches half-way).
2. 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.
3. 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.
4. 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.
• You don't know it's burned "exactly half-way" as in step 1 you only know it's "approximately half-way". You could get close, by approximating, but it wouldn't be exact. – Duncan Jul 29 '14 at 16:36
• You know its halfway because you can measure that the length of the lit candle is half of the unlit - accuracy is down to you sense of perception and skill at retaining the first mark/measurement;). Assume you have an infinitely thin fingernail, could hold the mark with absolutely no movement and had perfect vision you could determine when the lit candle is exactly half the height of the unlit candle (well assuming that the candle also burnt level but that type of problem applies to burning a candle at both ends too :p). – Alan Jul 29 '14 at 16:56