Without stoppages a bus averages 54 km/h, and with stoppages it averages 45 km/h. For how many minutes in each hour does the bus stand still?

In one hour of clock time the bus manages only 45 km, and covering 45 km at its running speed of 54 km/h takes 45/54 hour = 50 minutes. The other 10 minutes are spent standing. (Equivalently, the 9 km lost each hour would take 9/54 hour = 10 minutes to cover. Quoting the 9 km/h shortfall as 9 minutes is the trap.)