## 282. Math Puzzle $11 / 88$

A household community wants to build a rectangular sandbox for the children. As a border, 84 cubic stones with an edge length of $20 \mathrm{~cm}$ are to be used.

How wide will the sand surface be if it is to have a length of $5 \mathrm{~m}$? What is the area of the sandbox?
$5 \mathrm{~m}=500 \mathrm{~cm}=25 \cdot 20 \mathrm{~cm}$

For the length of the sand surface, 25 stones are needed, and for both lengths, 50 stones are needed. The two corner stones on each side do not play a role in calculating the effective digging area.

$84-50=34 ; 34: 2=17 ; 17-2=15$; Now it holds: $15 \cdot 20 \mathrm{~cm}=3 \mathrm{~m}$

The sand surface will be $3 \mathrm{~m}$ wide. The area is $15 \mathrm{~m}^{2}$.