Answer:
5.9 mph
Explanation:
The boat's speed is 15 mph
Given the current's speed is x, then
Boat's speed going upstream: 15 - x
=> time going upstream = 130/(15 - x)
Boat's speed going downstream: 15 + x
=> time going downstream = 130/(15 + x)
Total time
130/(15 - x) + 130/(15 + x) = 20.5
130(15 + x) + 130(15 - x ) = 20.5(15 + x)(15 - x)
130(15 + x + 15 - x) = 20.5(225 - x^2)
20.5(225 - x^2) = 130(30)
225 - x^2 = 3900/20.5
x^2 = 225 - 3900/20.5
x = square root of (225 - 3900/20.5)
x = ±5.895 or ±5.9
since speed can't be negative, speed of current is 5.9