Suppose you have ten slow matches (also called match cord) and a lighter. If a slow match is lighted, it will burn for exactly one hour. The slow matches do not burn at constant speed, in other words it can happen that the first half of the slow match is burned in 15 minutes, while the remaining part will take three quarters. How can you measure exactly 45 minutes using these slow matches, and what is the minimum of slow matches you need for this?

Light simultaneously a slow match on both ends and one slow match on only one end. The slow match which is lit on both sides will burn for exactly half an hour. If that slow match is burned, lit the non-burning side of the other slow match. Then that slow match will still burn for a quarter. After this slow match is burned, exactly 45 minutes have passed.

So you only need two slow matches to measure three quarters.