16. The answer is (B).

Let $x$ be the number of Andrea's classmates. The amount he expects to spend is $13 x$ cents, while the amount he actually spends is $10 x$ cents. The difference between these amounts is exactly the cost of 6 candies, which is 60 cents. Consequently, $x$ can be determined by solving the equation $13 x - 10 x = 60$, or $3 x = 60$, from which $x = 20$. The correct answer is $\mathbf{B}$.
16) Andrea enters a store with the exact amount of money to buy a candy for each of her classmates, at a price of thirteen cents each. However, the price of the candies has dropped to ten cents each, and Andrea buys six more candies than planned, using up all the money she had. How many classmates does Andrea have?
(A) 18,
(B) 20,
(C) 21,
(D) 23
(E) 24