Time taken by a boat to go 72 km upstream is 2 hours more than the time taken by it to go downstream. If the ratio of speed of boat in still water to the speed of stream is 5: 1, then find the speed of boat in still water.

Correct option is C

Information Given:

Distance = 72 km

Time upstream is 2 hours more than time downstream

Ratio of speed of boat in still water to speed of stream is 5:1

Formulas Used:

Time = Distance ÷ Speed

Downstream speed = Speed of boat in still water + speed of current

Upstream speed = Speed of boat in still water - speed of current

Basic Explanation:

Let the speed of boat in still water and the speed of stream be 5x and x respectively

ATQ,

​$$\frac{72}{5x - x} - \frac{72}{5x + x} = 2 \\$$

$$\frac{18}{x} - \frac{12}{x} = 2 \\$$​​​

x = 3

So required speed = 15 km/hr