show me how to work is problem.

Question

A student has earned scores of 87, 81, and 88 on the first 3 of 4 test.  If the student wants an average of exactly 87, what score must she earn on the forth test

Answer 1

Problem: show me how to work is problem.
A student has earned scores of 87, 81, and 88 on the first 3 of 4 test.  
If the student wants an average of exactly 87, what score must she earn on the forth test

To average 87 after four tests, the total score must be 87 * 4 = 348
So far, the total score is 87 + 81 + 88 = 256
The next score must be 348 - 256 = 92

She must earn a 92 on the fourth test.