John can travel 8 miles upstream in the same time it takes him to go 12 miles downstream. His boat goes 15 mph in still water. What is the rate of the current/
(Vs - Vc)*t = 8 Miles.
(15 - Vc)t = 8.
Eq1: 15t - Vc*t = 8.
(15 + Vc)*t = 12.
Eq2: 15t + Vc*t = 12.
Add Eqs #1 and #2:
Sum: 30t = 20.
t = 2/3 s.
In Eq2, replace t with 2/3 s. and solve for Vc.
Vc = 3 m/s.