At a bus stop, frequency of bus is $20$ minutes. A passenger coming at random time write their waiting time. What is average waiting time? > The frequency of bus coming to a bus stop is $20$ minutes. Passengers come there at random time. They write the time they waited for the bus, as soon as the bus arrives they board the bus. What is the average of waiting time of all the passengers?

Passengers could come $20$ minutes early, or $0$ minutes early, and from this problem, we can assume they are evenly scattered in between.

Then, the average waiting time must be $10$ minutes.