Let t represent Yesenia's score on the third test. If her average is 79 for the three tests, then]
.. (91 +73 +t)/3 = 79
.. 164 +t = 237
.. t = 237 -164 = 73
Yesenia's score on the third test is 73.
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You can also look at points relative to the average. On her first test, Yesenia scored 12 points above her average. On her second test, 6 points below her average. In order to make the total relative to the average come out to be zero, she must have also scored 6 points below average on the third test, or 79 -6 = 73.
