math
posted by keke .
mike has an average of 89 on the five tests he has taken so far this semester. what grade must mike earn on the next test to raise his average grade exactly to 90?

5 * 89 = 445
600 * 0.9 = 540
540  445 = 95 
( x 1 + x 2 + x 3 + x 4 + x 5 ) / 5 = 89 Multiply both sides by 5
x 1 + x 2 + x 3 + x 4 + x 5 = 89 * 5 = 445
( x 1 + x 2 + x 3 + x 4 + x 5 + x 6 ) / 6 = 90
( 445 + x 6 ) / 6 = 90 Multiply both sides by 6
445 + x 6 = 6 * 90 = 540
x 6 = 540  445 = 95