The processes to rent a car in a certain car rental company involves: customer registration,

billing, and car inspection. The processing times for these three processes are $6$ minutes, $4$

minutes, and $15$ minutes, respectively. The car rental company allocates $1$ staff for

registration, $1$ staff for billing service, and $2$ staff for car inspection. Assume all staff are

working at full utilisation rates.

Solve to find the number of customers the store can process per hour. Which process is the

bottleneck? What can be done to ease the bottleneck?

Support your answer with necessary calculations.