Tim tiene que cronometrar un cuarto de hora exacto. Para ello utiliza dos mechas que arden durante una hora exacta, pero no de manera uniforme. Esto no significa que la mitad de una mecha se queme necesariamente después de media hora exacta.
¿Cómo consigue Tim cronometrar exactamente 15 minutos con las dos mechas?
Tim must light one fuse at both ends and the other at one end only. When the first fuse has burned down, he knows that the other one has to burn for another half hour. By lighting the other fuse at the second end as well, he can shorten this time by half and therefore time exactly 15 minutes.