A boat running downstream covers a distance of 30 km in 2 hours. While coming back the boat takes 6 hours to cover the same distance. If the speed of the current is half of that of the boat, then what is the speed of the boat in kmph?

Downstream speed = 30/2 = 15 kmph. Upstream speed = 30/6 = 5 kmph. Let boat speed = b, current speed = c. Downstream: b + c = 15. Upstream: b - c = 5. Given c = b/2 (current is half of boat speed). Substituting: b + b/2 = 15 gives 3b/2 = 15, so b = 10 kmph. Check: c = 5 kmph, which satisfies both equations. Option C is correct.