Phil Dunphy has two appointments to show a house to prospective buyers. The first appointment is at 3 PM and the second is at 3:30 PM. The amounts of time that appointments last are independent exponential random variables with mean 30 minutes. Phil always finishes one appointment before starting the next one. Assuming that both buyers are on time, what is the expected amount of time that the buyer scheduled for 3:30 will spend at the house?