1. Determine the GCD of the numbers 40, 60, and 20.
\text{GCD}(40, 60, 20) = 20
2. Divide each of the quantities by the GCD to find how many pieces each student will get.
For chocolate bars:
\frac{40}{20} = 2 \text{ chocolate bars per student}
For candies:
\frac{60}{20} = 3 \text{ candies per student}
For wafers:
\frac{20}{20} = 1 \text{ wafer per student}
Thus, each student will get 2 chocolate bars, 3 candies, and 1 wafer.
Answer: 20 students can each get 2 chocolate bars, 3 candies, and 1 wafer.