\(V_d = 15 mph\): Velocity downstream
\(V_u = 9 mph\): Velocity upstream
\(V_c \): Velocity of the current
\(V_s \): Velocity in still water
\(V_d = V_s + V_c\)
\(15 = V_s + V_c\)
\(V_u = V_s - V_c\)
\(9= V_s - V_c\)
By elimination eliminate \(V_c\)
\(24 = 2V_s\)
\(V_s = 12 mph\)
\(V_c = 15-12 = 3 mph\)
Thus the rate of the boat in still water is 12 mph and the rate of the current is 3 mph