Answer:
21 mph
Step-by-step explanation:
The relationship between time, speed, and distance is ...
time = distance/speed
The speed downstream is b+7 if the boat's speed is b. The speed upstream is b-7, and the times in each direction are equal. This means ...
20/(b+7) = 10/(b-7) . . . . . . time in each direction are the same
20(b-7) = 10(b+7) . . . . . . . cross multiply
(20-10)b = 7(10+20) . . . . . add 7(20)-10b
b = 7(20 +10)/(20 -10) = 21 . . . . mph
The speed of the boat in still water is 21 mph.
