Question

On his first two social studies tests, Billy made an 86 and a 93. What grade must Billy make on the third test to have an average of 90 for all three tests?

Answer 1

an average is calculated by
(T1 + T2 + T3)
-------------------
3
so we have
86 + 93 + x
---------------- = 90
3
then you just solve for x.

86 + 93 + x = 270
270 - 86 - 93 = x

Billy must score a 91.

Answer 2

90 * 3 = 270

86 + 93 = 179

270 -179 = 91

answer: He needs to make 91 on the third test to have an average of 90 for all three tests