A city bus has an average speed of 20 km/h if it doesn't stop anywhere. But if it stops in-between the average speed drops to 15 km/h. How many minutes does it stop in 1 hour?
Due to stoppages, it covers a less distance of 20 - 15 = 5 in one hour. The time taken for that distance would be the wastage due to stopping = $5/20$ × 60 = 15 mins.