400=3.14*20*r^2

Solution for 400=3.14*20*r^2 equation:

Simplifying
400 = 3.14 * 20r2

Multiply 3.14 * 20
400 = 62.8r2

Solving
400 = 62.8r2

Solving for variable 'r'.

Move all terms containing r to the left, all other terms to the right.

Add '-62.8r2' to each side of the equation.
400 + -62.8r2 = 62.8r2 + -62.8r2

Combine like terms: 62.8r2 + -62.8r2 = 0.0
400 + -62.8r2 = 0.0

Add '-400' to each side of the equation.
400 + -400 + -62.8r2 = 0.0 + -400

Combine like terms: 400 + -400 = 0
0 + -62.8r2 = 0.0 + -400
-62.8r2 = 0.0 + -400

Combine like terms: 0.0 + -400 = -400
-62.8r2 = -400

Divide each side by '-62.8'.
r2 = 6.369426752

Simplifying
r2 = 6.369426752

Take the square root of each side:
r = {-2.523772326, 2.523772326}