In down stream a boat goes 36 k.m. and in upstream it goes 35 k.m. and takes 11 hours. If boat goes 45 k.m. in down stream and 40 k.m. in upstream it takes 13 hours. Then find the speed of boat in still water and speed of stream?

Let boat speed = b, stream speed = s. Downstream speed = b+s, upstream = b-s. First trip: 36/(b+s) + 35/(b-s) = 11. Second trip: 45/(b+s) + 40/(b-s) = 13. Option C (7, 2) gives: Trip 1: 36/9 + 35/5 = 4 + 7 = 11 ✓. Trip 2: 45/9 + 40/5 = 5 + 8 = 13 ✓. So b=7 km/hr, s=2 km/hr is correct. Option A would give 36/7 + 35/3 ≈ 5.14 + 11.67 = 16.81 (wrong).