Final answer:
To find the average of three test scores with two known scores (77 and 96) and one unknown score S, the formula is: Average = (173 + S) / 3.
Step-by-step explanation:
To express the average of three test scores when the first two scores are known (77 and 96), and the third score S is unknown, we use the formula for finding the average (mean) of three numbers. In this case, the formula is:
(First Score + Second Score + Third Score) / 3 = Average
Substituting the given scores into the formula, we get:
(77 + 96 + S) / 3 = Average
We can simplify this to:
Average = (173 + S) / 3
This equation now expresses the average test score in terms of the variable S.