The postmaster at the Oak Hill Post Office expects that the mean arrival rate of people to her customer counter will soon increase by 50% due to a large apartment complex being built. Currently, the mean arrival rate is 13 people per hour. The postmaster can serve an average of 25 people per hour. By what percentage must the postmaster's mean service rate increase when the apartment complex is completed in order that the average time spent at the post office remains at its current value? (Assume Poisson arrivals and exponential service times.)

What is the % change needed. Please show work in Excel if possible.