A can of paint will cover 400 square feet. What is the length of the edge of the largest square that can be painted with this can of paint?

For the area of a circle is πr² as you stated so all you do is set 400 equal to πr² and solve for r.

400=πr²/π
127.32=r²
√127.32)= r
r=11.3

Same thing for this except you use 800 instead of 400 so.

800=πr²
254.64=r²
√254.64)= r
r=15.9 or 16.0 for rounding purposes.