Mike has scores of 87, 81, and 74 on his first three algebra tests. What score must he get on the?

First you set the fouth test score = x (you don't know the acutal score yet)
Then you know First second and third tests are 87, 81, 74
When you count fourth test score is "x"
Average for 4 tests is 85 which means "(87+81+74+x) "total score
__________ devided by 4(because 4 tests)
4
= 85
so the equation looks like this
(87+81+74+x)/4 = 85
now you mutiply 4 on both side first, to get rid of 4
you will get
87+81+74+x =340
then you subtract both side by 87
you will get 81+74+x=253
then you subtract both side by 81
you will get 74+x=172
then you subtract both side by 74
you will get x=98
Therefore you need to score 98 points( or higher) on the fourth test to get the average of 85 points or higher