Find the lowest score for the average. Let that number = x
There are 5 tests in all
(88 + 84 + 88 + 91 + x)/5 = 90
First, multiply 5 to both sides
(88 + 84 + 88 + 91 + x)/5(5) = 90(5)
88 + 84 + 88 + 91 + x = 450
Next, Combine like terms
351 + x = 450
Next, isolate the x. Subtract 351 from both sides
351 (-351) + x = 450 (-351)
x = 450 - 351
x = 99
99 is your answer
hope this helps