When the quantity rises from 1600 to 5600, the unit price rises from $220 to $270, so an increase in 4000 in the quantity corresponds to an increase of $50 in the unit price. 4000 is 80 times 50, meaning that for every dollar increase in unit price there is an increase in quantity of 80. So we start with Q=80P where Q=quantity and P=unit price. 1600 is not equal to 80 times 220; 80 times 220=17600, so we need to subtract 16000 to get Q=1600, therefore we can write Q=80P-16000. This is also true if P=270, since 80P=21600, less 16000 is 5600, the quantity. Now we have an equation we can use to find Q when P=320: Q=80×320-16000=9600.