Sandra's scores on the first 4 tests were 87, 92, 76 and 89. What is the minimum score she needs to make on the fifth test so that her average is at least an 80.

Let the score on fifth test be x

Mean test score = (87 + 92 + 76 + 89 + x)/5

Then (87 + 92 + 76 + 89 + x)/5 >= 85

(344 + x)/5 > = 85

Multiply by 5 on each side, we will get

344 + x >= 85*5

344 + x>=425

x>= 425 - 344

x>=81