If Chandu covers 285 km in a boat in 45 hours against the stream while he takes 18 hours to cover the same distance downstream, then find the speed of the stream.

The correct option is 4.75 km/h.

Let's break down the solution step-by-step to find the speed of the stream:

Step 1: Understand the given details
Distance covered by the boat, $d=285\text{km}$
Time taken to travel upstream (against the stream), $t_u=45\text{hours}$
Time taken to travel downstream (with the stream) for the same distance, $t_d=18\text{hours}$

Step 2: Calculate the upstream and downstream speeds
The speed upstream ($v_u$) is the distance divided by the upstream time:
$v_u=\frac{285}{45}=6.333\text{km/h}$

The speed downstream ($v_d$) is the distance divided by the downstream time:
$v_d=\frac{285}{18}=15.833\text{km/h}$

Step 3: Find the speed of the stream
The speed of the stream ($s$) is given by the formula:
$s=\frac{v_d-v_u}{2}$

Substitute the calculated upstream and downstream speeds into the formula:
$s=\frac{15.833-6.333}{2}$
$s=\frac{9.5}{2}=4.75\text{km/h}$

Therefore, the speed of the stream is 4.75 km/h.