Interview : Expected time to see a bus

Suppose a bus is running on a loop route. It takes 10 mins for the bus to finish the route. If a guy arrives at a bus stop at uniformly random time. What is the expected bus waiting time?

Sol: image uniformly put a point at a line with length 10, so the expected waiting time is 5.



Now suppose there is a ice cream shop at the bus loop. Whenever the bus is close to the ice cream shop, the driver will flip a coin and decide whether to go eating an ice cream or not. It takes 10 mins for the driver to eat the ice cream. What is the expected bus waiting time if a guy arrives at a bus stop uniformly.

Sol: the key is to consider what uniform arriving time means. The bus is running either on a 10min-loop or a 20min-loop, each with probability 1/2. The bus keeps running, and the guy can arrive at any time between 0am - 24pm, uniformly. The time for the bus to be in a 20min-loop is two times larger than the time for a 10min-loop. So each day 2/3 of the time the bus is in a 20min-loop, and 1/3 of the time the bus is in a 10min-loop.

Thus, the probability for the guy to arrive when the bus is in a 20min-loop is 2/3. Expected waiting time is 10 for the 20min-loop and 5 for the 10min-loop. So the combined expected waiting time is

2/3 * 10 + 1/3 * 5