Let \(V_b\) be the speed of boat on still water

Let \(V_w\) be the speed of flowing water

UPSTREAM - This means that the boat is moving againts the current (Flow of water).

Thus; \(V_u = V_b - V_w\). As the boat moves up, the water is reducing its speed.

DOWNSTREAM- This means that the boat is moving with the current (Flow of water).

Thus; \(V_u = V_b + V_w\)

\(V_u = \frac{distance travelled, s}{Time of travel, t} = \frac{60}{15}\)

\(V_u = 4 mph\)

\(V_d = \frac{60}{6} = 10 mph\)

\(4 = V_b - V_w\)

\(10 = V_b + V_w\)

Add the two equations: \(14 = 2 V_b\)

\(V_b = 7 mph\) Answer