Think about radioactive half-life. Half the atoms will decay in X minutes. Half the remainder in 2X minutes. Half the remainder in 3X etc. Likewise you'll serv

Think about radioactive half-life. Half the atoms will decay in X minutes. Half the remainder in 2X minutes. Half the remainder in 3X etc. Likewise you'll serve half your customers within X minutes. Half the remainder within 2X, half the remainder within 3X etc.