A computer store sells two types of computers, laptops and desktops. The supplier demands that at least 120 computers be sold a month. Experience shows that most consumers prefer laptops, but some business customers require desktops. The result is that the number of laptops sold is at least twice of the number of desktops. The store pays its sales staff a $60 commission for each laptop, and a $40 commission for each desktop. How many of each type must be sold to minimize commission to its sales people? What is the minimum commission?