Phil played golf four days last week. His average score was 82. His total score for the first three days was 239. What was his score for the 4th day?
So the average score for for days would be calculated like this: (Sum of the scores) / (# of days) or (Sum of the scores for the four days) / (4 days).
So you can add this up like (S1+S2+S3+S4) / 4 = average.
Well you know some of these things and can make substitutions.
S1+S2+S3=239 and the average is 82. So,
(239+S4) / 4 = 82
Solve for S4