math
posted by bobby .
Jim has grades of 84, 65, and 76 on three math tests. What grade must he obtain on the next test to have an average of exactly 80 for the four tests?

Well first you add up the currently known grades: 84, 65, and 76. (known grades)
Next multiply 80 by 4 to get the total amount for his test scores. (wanted average)
Finally subtract the total for his known grades from his wanted average. 
4 * 80 = 320
84 + 65 + 76 = 225
320  225 = 95