There are 10 buse

Given: 10 buses running between Delhi and Agra.

To Find: Number of ways a man can go from Delhi to Agra and return by a different bus.

There are 10 buses running between Delhi and Agra so there are 10 different ways to go from Delhi to Agra. The man cannot return from the same bus he went so number of ways are reduced to 9.

These second event occur in completion of first event so there are:10 × 9=90 ways in which a man can go from Delhi to Agra and return by a different bus.