To calculate the average speed of a journey, we need to use the formula:

Average Speed = (Total distance traveled) / (Total time taken)



Let's assume that the distance traveled by the man in each direction is 'd' kilometers.

Therefore, the total distance traveled by the man in the whole journey is:

Total Distance = 2d

This is because he travels the same distance twice, once in each direction.



Let's assume that the time taken by the man to travel 'd' kilometers at a speed of 20 km/hr is 't' hours.

Therefore, the time taken by the man to travel 'd' kilometers at a speed of 30 km/hr is:

t1 = d / 20

t2 = d / 30

The total time taken by the man in the whole journey is:

Total Time = t1 + t2



Now, we can use the formula of average speed to calculate the average speed of the whole journey:

Average Speed = (Total distance traveled) / (Total time taken)

Substituting the values of total distance and total time, we get:

Average Speed = (2d) / (t1 + t2)

= (2d) / (d/20 + d/30)

= (2d) / [(3d + 2d)/60]

= (2d) / (5d/60)

= 24 km/hr

Therefore, the average speed of the man in the whole journey is 24 km/hr.