Speed of stream

Let stream speed be $v\text{ km/hr}$. Upstream speed $= 5 - v$, Downstream speed $= 5 + v$.

Given: $t_{\text{up}} = 3 \cdot t_{\text{down}}$ $\frac{20}{5 - v} = 3 \times \frac{20}{5 + v}$ $5 + v = 3(5 - v) \implies 5 + v = 15 - 3v \implies 4v = 10 \implies v = 2.5\text{ km/hr}$