Answer:
20

Explanation:
The diagonal splits the square into 2 right triangles
Using Pythagoras' identity on one of the right triangles
With legs 10
and hypotenuse the diagonal d, then
d² = (10
)² + (10
)² = 600 + 600 = 1200 ( take square root of both sides )
d =
=
=
×
= 20
