Answer: the speed of the current is 10 mph.
Step-by-step explanation:
Let x represent the speed of the current.
Janet’s boat will go 10 miles per hour in still water. This means that the total speed at which she went downstream is (10 + x) mph.
Time = distance/speed
If he can go 120 miles downstream, then the time taken to go downstream is
120/(10 + x)
The total speed at which she went upstream is (10 - x) mph. If at the same time, she goes 40 miles upstream, then the time taken to go downstream is
40/(10 - x)
Since the time is the same, it means that
120/(10 + x) = 40/(10 - x)
Cross multiplying, it becomes
120(10 - x) = 40(10 + x)
1200 - 120x = 400 + 40x
120x + 40x = 400 + 1200
160x = 1600
x = 1600/160
x = 10 mph
