S1:
We could have the jersey numbers 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, the average of which is about 50, or we could have 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, the average of which is 91. Conflicting results, so insufficient. The tricky part here, I think, is that the question wants you to assume the jersey numbers are consecutive ... but they might not be!
S2:
The smallest possible set is 78 through 98, the average of which is greater than 80. Since our smallest average > 80, this is sufficient.
