The buses are departed after every 20 min and a man who was going away from the bus depot get the buses after every 24 min. Find the speed of buses if the speed of man is 30 km/hr.

180 km/hr

The distance covered by bus in 4 minutes is equal to the distance covered by man in 24 minutes.

                 B    :     M

Time          4    :     24

Speed       24    :     4

Speed        6     :    1

                 ↓           ↓

               ×30       ×30

                 ↓           ↓

               180       30