Average problem involving missing test score {ACT}

Kaleb earned 85, 95, 93, and 80 points on the 4 tests, each worth 100 points, given so far this term.  How many points must he earn on his fifth test, also worth 100 points, to average 90 points for the 5 tests given this term?
A.)  87
B.)  88
C.)  90
D.)  92
E.)  97

  sum of terms
--------------------- = average
number of terms

85 + 95 + 93 + 80 + x
---------------------------- = 90   {substituted x for missing score}
               5

353 + x
---------- = 90   {combined like terms in numerator}
     5

353 + x = 450   {multiplied each side by 450}

x = 97

97 points must be earned on 5th test

E.)


- Algebra House