We have two candles that we know take 1 hour each to consume completely and also do it uniformly.
How can we measure 15 minutes?
We light one of the candles at both ends simultaneously and the second candle only by one of them. Once the candle burning at both ends is completely consumed, 30 minutes will have elapsed. At that time half of the second candle will have been consumed so if we light it at the other end it will allow us to measure 15 minutes.