A man goes from C to D at 40 km/h and he returns from D to C at x km/h. If the average speed of the man for the whole journey is 60 km/h, then what is the value of x?

Let's assume the distance between C and D is 'd' km.

A man goes from C to D at 40 km/h and he returns from D to C at x km/h. If the average speed of the man for the whole journey is 60 km/h.

$$Average\ speed=\frac{\left(total\ distance\right)}{total\ time}$$

$$60 =\frac{\left(d+d\right)}{\frac{d}{40}+\frac{d}{x}}$$

$$60 =\frac{\left(2d\right)}{d\left(\frac{1}{40}+\frac{1}{x}\right)}$$

$$60 =\frac{2}{\frac{x+40}{40x}}$$

$$60=\frac{80x}{x+40}$$

$$3 = \frac{4x}{x+40}$$

3x+120 = 4x

4x-3x = 120

x = 120