Answer:
98
Step-by-step explanation:
Let x be the next test score
87, 92, 90, 84, 88, and 91, and x
We want to average them and the average to 90
(87+ 92+ 90+ 84+ 88+ 91+x)/7 = 90
Multiply each side by 7
(87+ 92+ 90+ 84+ 88+ 91+x)/7*7 = 90*7
(87+ 92+ 90+ 84+ 88+ 91+x) = 630
Combine like terms
x+532=630
Subtract 532
x = 630-532
x= 98