A man covers the journey from station A to station B at a uniform speed of 36 kmph and returns to A with a uniform speed of 45 kmph, his average speed for the whole journey is:

Answer: Option A

Average speed for this case is given by,
$$\frac{2xy}{x+y},$$   where X and Y are two different speeds.
Average speed,
$$\begin{array}{rl}& =\frac{2\times 36\times 45}{36+45}\\ & =\frac{36\times 90}{81}\\ & =40\text{kmph}\end{array}$$