Plz me really really fast 2 mins left

C

Step-by-step explanation:

The diagonal of the square creates 2 right triangles with the diagonal as the hypotenuse.

Using Pythagoras' identity in either of the right triangles.

The square on the hypotenuse is equal to the sum of the squares of the other 2 sides.

Let x be the side of the square, then

x² + x² = 40², that is

2x² = 1600 ( divide both sides by 2 )

x² = 800 ( take the square root of both sides )

x =

  =

=  = 10

  = 10 ×

  = 10 × 2 = 20 → C