Ann's grade is the average of four scores: Three chapter tests and a final that counts as two chapter tests. She scored 72, 90, and 75 on the chapter tests. What score does she need to have on the final to have a grade at least 80?

1) 80*5 = 400

2) 400 - ( 72+90+75) = 163

3) 163/2 = 82 rounded up.

The answer 82.