Correct Answer - Option 4 : 4.5
Given:-
Time for 88 kms downstream and 65 km upstream = 9 hours
Time for 121 kms downstream and 97.5 km upstream = 13 hours
Formula:-
Speed = Distance/Time
Downstream speed = u + v
Upstream speed = u - v
Boat speed = ½ (D + U)
Stream speed = ½ (D - U)
U and v are boat and stream speeds and D and U downstream and upstream speeds.
Calculation:-
88/(u + v) + 65/(u - v) = 9
121/(u + v) + 97.5/(u - v) = 13
Let 1/(u + v) = x and 1/(u - v) = y
88x + 65y = 9
121x + 97.5y = 13
Solving both, x = 1/22 and y = 1/13
Or U = 13 and D = 22
V = ½ (22 – 13) = 4.5 km/hr