20=x(0.8x)

Solution for 20=x(0.8x) equation:

Simplifying
20 = x(0.8x)

Remove parenthesis around (0.8x)
20 = x * 0.8x

Reorder the terms for easier multiplication:
20 = 0.8x * x

Multiply x * x
20 = 0.8x2

Solving
20 = 0.8x2

Solving for variable 'x'.

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

Add '-0.8x2' to each side of the equation.
20 + -0.8x2 = 0.8x2 + -0.8x2

Combine like terms: 0.8x2 + -0.8x2 = 0.0
20 + -0.8x2 = 0.0

Add '-20' to each side of the equation.
20 + -20 + -0.8x2 = 0.0 + -20

Combine like terms: 20 + -20 = 0
0 + -0.8x2 = 0.0 + -20
-0.8x2 = 0.0 + -20

Combine like terms: 0.0 + -20 = -20
-0.8x2 = -20

Divide each side by '-0.8'.
x2 = 25

Simplifying
x2 = 25

Take the square root of each side:
x = {-5, 5}