A boat travels along a river at a speed of $v_{\text{boat}} = 15$ km/h relative to the water. The speed of the current is $v_{\text{cur}} = 3$ km/h. Find the speed of the boat relative to the bank downstream and upstream.
Velocities along the same direction add up. Downstream:
$$v_{\text{down}} = v_{\text{boat}} + v_{\text{cur}} = 15 + 3 = 18 \text{ km/h}$$
Upstream:
$$v_{\text{up}} = v_{\text{boat}} - v_{\text{cur}} = 15 - 3 = 12 \text{ km/h}$$
Downstream $18$ km/h, upstream $12$ km/h