Let's say the number of vacation days taken by each employee last year are a, b, c, d, and e, respectively.

The prompt tells us that (a+b+c+d+e)/5 = 16, or a + b + c + d + e = 80. So we had 80 vacation days last year.

From S1, we can say that the number of vacation days taken by each employee this year are 1.5a, 1.5b, 1.5c, 0.5d, and 0.5e. We can't find the sum of these numbers, however: we only know that a + b + c + d + e = 80, and we're doing DIFFERENT things to the five variables. We need more equations: NOT SUFFICIENT.

From S2, we know that we have 20 more vacation days in total than we had last year. (We're adding 30 for the three employees, and subtracting 10 for the other two employees.) Since we had 80 vacation days last year, we must have 100 vacation days this year. Hence the average is 100/5, or 20. SUFFICIENT!