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. How do you do it?
There are two possible solutions (and maybe more).
Solution 1: Lie the candles horizontally next to each other, but with the wicks at opposite ends. Light both the candles. 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.
Solution 2: First light up the two ends of the first candle. When it has fully burned, light up one end of the second candle. First candle takes 30 mins to burn out and second candle will take 60 mins – total of 90 minutes.