Mona wrote 7 tests, and her test average is 93. there will be one more test before the end of the year. what is the lowest grade that mona can get for her last test so that her test average is 90 or more?

For this case, the first thing we are going to do is assume that all the tests are worth the same.
 Then, we define a variable:
 x: score of Mona's last test
 We write now the inequality that models the problem:
 
 From here, we clear the value of x: 
 
 the lowest grade that Mona can get for her last test so that her test average is 90 or more is:
 x = 87

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