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