Time taken by a boat to cover (D-11) km in upstream is 5 times of the time taken by boat to cover (D-21) km in downstream. If ratio of speed of current to speed of boat in downstream is 1 : 3 and boat can cover (D-8) km in upstream in 14 hours, then, find speed of boat in still water?

Let boat speed be B and current speed be C. Downstream speed = B+C, Upstream = B-C. Ratio C/(B+C) = 1/3 implies 3C = B+C, so B = 2C. Upstream speed = B-C = C. Downstream speed = 3C. Time upstream = (D-11)/C. Time downstream = (D-21)/3C. Given (D-11)/C = 5 * (D-21)/3C, so 3(D-11) = 5(D-21), 3D - 33 = 5D - 105, 2D = 72, D = 36. Upstream time = (36-8)/C = 14, so 28/C = 14, C = 2. B = 2C = 4 kmph.