The area of a circle is given by
[tex]A=\pi r^2[/tex]
We can solve this equation for r: divide both sides by pi:
[tex]r^2=\dfrac{A}{\pi}[/tex]
Consider the (positive) square root of both sides:
[tex]r = \sqrt{\dfrac{A}{\pi}}[/tex]
So, in your case, we have
[tex]r = \sqrt{\dfrac{40}{\pi}}\approx 3.57[/tex]
