20=r(r-8)

Solution for 20=r(r-8) equation:

Simplifying
20 = r(r + -8)

Reorder the terms:
20 = r(-8 + r)
20 = (-8 * r + r * r)
20 = (-8r + r2)

Solving
20 = -8r + r2

Solving for variable 'r'.

Reorder the terms:
20 + 8r + -1r2 = -8r + 8r + r2 + -1r2

Combine like terms: -8r + 8r = 0
20 + 8r + -1r2 = 0 + r2 + -1r2
20 + 8r + -1r2 = r2 + -1r2

Combine like terms: r2 + -1r2 = 0
20 + 8r + -1r2 = 0

Factor a trinomial.
(10 + -1r)(2 + r) = 0

Subproblem 1
Set the factor '(10 + -1r)' equal to zero and attempt to solve:

Simplifying
10 + -1r = 0

Solving
10 + -1r = 0

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

Add '-10' to each side of the equation.
10 + -10 + -1r = 0 + -10

Combine like terms: 10 + -10 = 0
0 + -1r = 0 + -10
-1r = 0 + -10

Combine like terms: 0 + -10 = -10
-1r = -10

Divide each side by '-1'.
r = 10

Simplifying
r = 10
Subproblem 2
Set the factor '(2 + r)' equal to zero and attempt to solve:

Simplifying
2 + r = 0

Solving
2 + r = 0

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

Add '-2' to each side of the equation.
2 + -2 + r = 0 + -2

Combine like terms: 2 + -2 = 0
0 + r = 0 + -2
r = 0 + -2

Combine like terms: 0 + -2 = -2
r = -2

Simplifying
r = -2
Solution
r = {10, -2}