Question 266578
First of all you need to add all 4 grades
65+78+85+82=310 (you Have)

If you took 5 tests your total would need to be 400 to receive and 80 average overall.  Because 400 divided by 5 equals 80

Now you would subtract the 400 you need minus 310 you have.
400-310 = 90.

You would need to score 90 on your next test