Hi,
D = r*t
Let (x+c) and (x-c) represent speed downstream and upstream respectively
Question states
7.5mi = 1.5hrs(x+c) solving for x
7.5/1.5 - c = x
5 -c = x
7.5(x-c) = 2.5hr
(x-c) = 2.5/7.5 substituting for x to solve for c
[(5-c) - c] = 1/3
5 - 2c = 1/3
15/3 - 1/3 = 2c
14/3 = 2c
c = 7/3 or 2 1/3mph the speed of the current
x = 2 2/3mph the speed of the boat in still water
CHECKING our Answer
3/2hr * 5mph = 15/2 = 7.5mi