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Question:

Lynn has 4 test scores in science class. Each test is worth 100 points, and Lynn has an 85% average. If Lynn scored 100% on each of the first 3 tests, what did she score on her 4th test?

A 40%
explanation

To calculate the average, add all of the scores and divide by the total number of scores. Use the variable x in place of the missing score.
Let the score of the 4th test be "x"
Because Lynn scored 100% on 3 test and the average score is 85%, we have:
(x + 100 + 100 +100)/4 = 85
(x + 300)/4 = 85
x+ 300 = 85 * 4
x + 300 = 340
x = 340 - 300
x = 40