$A$ can row $15\text{km/h}$ in still water. It takes him twice as long to row upstream as to row downstream. Find the rate downstream.

Let the speed of the stream be $x\text{km/h}$.Speed downstream ($v_d$) = $15+x$.Speed upstream ($v_u$) = $15-x$.Time taken is inversely proportional to speed for a fixed distance ($t=D/v$).According to the problem, $t_{up}=2\times t_{down}$.
$$\frac{D}{15-x}=2\times \frac{D}{15+x}$$
$$15+x=2(15-x)$$
$$15+x=30-2x⟹3x=15⟹x=5\text{km/h}$$
The rate downstream is $v_d=15+5=20\text{km/h}$.