If Ishwar covers 721 km in a boat in 42 hours against the stream and he takes 15 hours with the stream, then find the speed of the stream.

Correct option is D

Given:

Distance covered against the stream = 721 km

Time taken against the stream = 42 hours

Time taken with the stream = 15 hours

Formula Used:

Upstream Speed (U) = $$\frac{\text{Distance}}{\text{Time against the stream}}$$​​

Downstream Speed (D) = $$\frac{\text{Distance}}{\text{Time with the stream}}$$​​

Speed of stream (S) = $$\frac{1}{2} \times (\text{Downstream Speed} - \text{Upstream Speed})$$​​

Solution:

Upstream Speed (U) = $$\frac{721}{42} \approx 17.1667 \, \text{km/h}$$​​

Downstream Speed (D) = $$\frac{721}{15} \approx 48.0667 \, \text{km/h}$$​​

Speed of Stream (S) = $$\frac{1}{2} \times (48.0667 - 17.1667)$$​​

​$$= \frac{1}{2} \times 30.9 = 15.45 \, \text{km/h}$$​​