A geometry problem by Baranee Tharan

If diagonal is 10 cm, then the side must be $\sqrt {50}$ hence the answer is 50.

The diagonal of a square is given by $d=x\sqrt{2}$ where $x$ is the edge length of the square. Substituting, we get

$10 = x\sqrt{2}$

Squaring both sides gives

$100 = x^2(2)$

Dividing both sides by $2$ gives

$x^2=\boxed{50}$