Answer:
17.5 mph
Step-by-step explanation:
Let the speed of the boat in still water be B mph
Let the rate of the current be R mph
When going upstream the effective speed is B-R mph. In terms of values given effective upstream speed = 42/3 = 14 mph
So,
B-R = 14 ... (1)
Similarly downstream speed is B + R (faster downstream)
B + R = 42/2 = 21 .... (2)
(1) + (2) ==> 2B = 35
B = 35/2 = 17.5 mph