A train completes a journey with a few stoppages in between at an average speed of 40 km per hour. If the train had not stopped anywhere, it would have completed the journey at an average speed of 60 km per hour. On an average, how many minutes per hour does the train stop during the journey?

1. (a)20 minutes per hour
2. (b)18 minutes per hour
3. (c)15 minutes per hour
4. (d)10 minutes per hour