James bond is a spy. He has to give signal to his senior after 90 minutes. He do not have any watch. He has two rods, each of which takes two h ours to burn if lit at one end. These rods are not homogeneous and some p arts of the rod burn more quickly than the other parts. He cannot cut the rod into h alf. What should he do to give the signal after 90 minutes.

• Set fire to both ends of one rod. At the s ame time, set fire to one end of the other rod. The rod lit from both ends will take one hour to burn completely (it is burning at twice the rate). Just as it finishes burning, the other rod would have burned for one hour with one hour remaining. Now light this rod from the other end as well, so that the remaining part burns in 30 minutes.
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