The speed of a boat in still water is 15 km/hr and speed of stream is 5 km/hr. The boat goes x km downstream and then returns back to the point of start in :

1. $\Big(\dfrac{x}{20} - \dfrac{x}{5}\Big)$ hrs

2. $\Big(\dfrac{x}{10} - \dfrac{x}{20}\Big)$ hrs

3. $\Big(\dfrac{x}{20} + \dfrac{x}{10}\Big)$ hrs

4. $\Big(\dfrac{x}{20} - \dfrac{x}{10}\Big)$ hrs

Speed of boat (downstream) = Speed of boat in still water + Speed of stream = 15 + 5 = 20 km/hr.

Speed of boat (upstream) = Speed of boat in still water - Speed of stream = 15 - 5 = 10 km/hr.

By formula,

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

Time taken by boat to go x km downstream = $\dfrac{x}{20}$

Time taken by boat to go x km upstream = $\dfrac{x}{10}$

Total time taken = $\Big(\dfrac{x}{20} + \dfrac{x}{10}\Big)$ hrs.

Hence, Option 3 is the correct option.