Answer:
For upstream: 12 mph
For downstream: 30 mph
Explanation:
Given that the speed of the boat in still water is 21mph and the speed of the current is 9mph
To find the speed of a boat both upstream and downstream,
For upstream, the boat is moving against the direction of the current. The speed will be:
21 - 9 = 12 mph
For downstream, the boat is moving toward the current direction. The two speed will be added together. That is,
21 + 9 = 30 mph
If the direction of the both is perpendicular to the direction of the current, then we will use pythagorean theorem
Speed = sqrt( 21^2 + 9^2 )
Speed = sqrt( 522 )
Speed = 22.85 mph