A small convenience store normally has one employee attending to the cash register. The average time between customer arrivals at the store is 5 minutes. The average service time at the cash register is 3 minutes. If more than two customers are standing at the register (including the customer being served), management has observed that other customers sometimes become upset and leave the store. What percentage of the time will this problem will occur?

about 30% about 40% about 20% about 50%?