3.38 Place a table of a certain height and two identical blocks of wood in the shape shown in Figure 1, and measure the length \( r \) to be 32 inches. When the two blocks of wood are rearranged as shown in Figure 2, the length \( s \) is measured to be 28 inches. The height of the table is
(A) 28 inches.
(B) 29 inches.
(C) 30 inches.
(D) 31 inches.
(E) 32 inches.
(37th American High School Mathematics Examination, 1986)
[Solution] Let the length of the block be $x$, the width be $y$, and the height of the table be $l$. According to the problem, we have
$$
\left\{\begin{array}{l}
x+l-y=r, \\
y+l-x=s .
\end{array}\right.
$$
(1) + (2) gives $2 l=r+s$,
$$
\therefore \quad l=\frac{1}{2}(32+28)=30 \text{ (inches). }
$$

Therefore, the answer is $(C)$.