3.18 Ann and Sue each bought a box of the same stationery. Ann used hers to write some one-page letters, and Sue used hers to write some three-page letters. Ann used up all the envelopes but had 50 sheets of paper left, while Sue used up all the paper but had 50 envelopes left. The number of sheets of paper in each box is
(A) 150.
(B) 125.
(C) 120.
(D) 100.
(E) 80.
(23rd American High School Mathematics Examination, 1972)
[Solution] Let $x$ and $y$ be the number of sheets of paper and envelopes in each box, respectively. According to the problem, we have
$$
\left\{\begin{array}{l}
x-y=50, \\
y-\frac{1}{3} x=50,
\end{array}\right.
$$

This gives $\frac{2}{3} x=100$, so the number of sheets of paper in each box $x=150$.
Therefore, the answer is $(A)$.