7. (a) which hr did more leave then arrive.....3:00 - 4:00......30 left and 18 arrived
(b) 75 customers in the store at 1:00.....30 entered, 12 left.....75 + 30 - 12 = 93, so at 2:00, there is 93 customers...14 entered, 8 left.....93 + 14 - 8 = 99...so at 3:00, there are 99 customers...18 entered, 30 left....99 + 18 - 30 = 87....so at 4:00, there were 87 customers in the store.....so, between 4:00 and 5:00, there are 87 customers that need to leave