A man takes 20 hours to go to a place and come back by walking both the ways. He could have saved 4 hours by riding both the ways. The distance covered in the whole journey is 36 miles. Find the average speed for the whole journey if he goes by walking and comes back by riding.

A

2 miles/hr

B

1 miles/hr

C

4 miles/hr

D

3 miles/hr