She will have to score 93.
(80+65+87+75+x)/5 = 80
307+x = 400
x =400-307=93
Another way, easier to do in your head:
Notice that 65 is 15 below 80, 87 is 7 above 80, and 75 is 5 below 80. The overall situation is 13 below 80 (because -15+7-5=-13). For a number to push the average back to 80, it must be 13 higher than 80. 80+13=93
