In a stream running at 2 kmph, a boat goes 10 km upstream and back again to the starting point in 55 minutes. Find the speed of the boat in still water.

Let boat speed in still water be b kmph. Upstream speed = b-2 kmph, downstream speed = b+2 kmph. Time upstream = 10/(b-2) hours, time downstream = 10/(b+2) hours. Total time = 10/(b-2) + 10/(b+2) = 55/60 hours. Solving gives b = 22 kmph. Option A (8 kmph) is too slow - the boat would barely move upstream. Option B (11 kmph) is a common error from incorrect equation setup. Option D (17 kmph) doesn't satisfy the time equation.