A boat takes 11 minutes and 22 minutes to cover a particular distance downstream and upstream respectively. If the speed of the boat in still water is 15 km/h, find the speed of the stream.

Let distance = D, stream speed = s. Downstream: D/(15+s) = 11 min. Upstream: D/(15-s) = 22 min. Dividing: (15-s)/(15+s) = 11/22 = 1/2. So 2(15-s) = 15+s → 30-2s = 15+s → 15 = 3s → s = 5. The key insight is upstream time is exactly double downstream time, making the stream half the boat's speed. Options A, B, C are too small.