25. A square piece of graph paper of side length $30 \mathrm{~mm}$ contains 900 smallest squares each of side length $1 \mathrm{~mm}$ each. Its four rectangular corners, denoted by $\mathrm{A}, \mathrm{B}, \mathrm{C}, \mathrm{D}$ in clockwise order, are cut away from the square piece of graph paper. The resultant graph paper, which has the shape of a cross, is shown in the figure below. Let $N$ denote the total number of rectangles, excluding all the squares which are contained in the resultant graph paper. Find the value of $\frac{1}{10} N$.
25. Answer: 14413
The number of rectangles (including squares)
$$
\begin{array}{l}
=(20+19+18+\ldots+2+1)(30+29+28+\ldots . .+1) \times 2-(20+19+18+\ldots .+1)^{2} \\
=151200 .
\end{array}
$$

The number of squares
$$
\begin{array}{l}
=(20 \times 30+19 \times 29+18 \times 28+\ldots . .+1 \times 11) \times 2-\left(20^{2}+19^{2}+18^{2}+\ldots .+1^{2}\right) \\
=7070 .
\end{array}
$$
$N=$ the number of rectangles less all squares $=151200-7070=144130$.

Hence $N / 10=14413$.