A boy can swim in still water at a speed of $10\text{ km/hr}$. If the speed of the current would have been $5\text{ km/hr}$, then the boy could swim $60\text{ km}$ downstream (along the flow) in how much time?

Given data:
* Speed of the boy in still water ($u$) = $10\text{ km/hr}$
* Speed of the stream/current ($v$) = $5\text{ km/hr}$
* Downstream distance ($d$) = $60\text{ km}$

The effective speed of the boy swimming downstream (along the flow of water) is:

$$\text{Downstream Speed }(S_{\text{down}})=u+v=10+5=15\text{ km/hr}$$

We know that:

$$\text{Time}=\frac{\text{Distance}}{\text{Speed}}$$

Substitute the values:

$$\text{Time}=\frac{60}{15}=4\text{ hours}$$