A motorboat travelling at full speed against a current goes 15 miles in one hour. Travelling at half speed with the current, it goes 12 miles in an hour. Find the rate of the current.
Eq1: Vm - Vc = 15.
Eq2: 0.5Vm + Vc = 12.
Sum: 1.5Vm = 27, Vm = 18 mi/h.
In Eq1, replace Vm with 18 and solve for Vc.