Answer:
2 mph
Step-by-step explanation:
Distance = speed × time
time = distance / speed
If x is the speed of the current, then 12 + x is the downstream speed and 12 − x is the upstream speed.
The time downstream is the same as the time upstream, so:
21 / (12 + x) = 15 / (12 − x)
Cross multiply and solve:
21 (12 − x) = 15 (12 + x)
252 − 21x = 180 + 15x
72 = 36x
x = 2
Let's check our answer. At 14 mph, he can travel 21 miles in 1.5 hours. At 10 mph, he can travel 15 miles in 1.5 hours.
So the speed of the current is 2 mph.
