16. (6 points) Master Zhang bought a certain number of apples at a price of 4 apples for 1 yuan, and then sold these apples at a price of 5 apples for 2 yuan. If he wants to make a profit of 15 yuan, how many apples must he sell? $\qquad$
【Solution】Solve: $15 \div(2 \div 5-1 \div 4)$
$$
\begin{array}{l}
=15 \div(0.4-0.25), \\
=15 \div 0.15, \\
=100 \text { (apples); }
\end{array}
$$

Answer: He must sell 100 apples.
Therefore, the answer is: 100.