Answer:
2 mph
Step-by-step explanation:
time = distance / speed
Let c represent the rate of the current. Then the upstream rate of the boat relative to the shore is (5c -c) = 4c, and the upstream time is ...
... 12/(4c) = 3/c.
Likewise, the downstream rate is (5c +c) = 6c, and the downstream time is ...
... 12/(6c) = 2/c.
The total time is the sum of upstream and downstream times:
... 3/c +2/c = 2.5
... 5/c = 2.5 . . . . . . combine terms
... 5/2.5 = c = 2 . . . multiply by c/2.5
