Question

20 = 2 · a
a = -10
a = 12
a = 18
a = -18

Explanation:

Step1: Isolate the variable a.

Divide both sides of the equation $20 = 2\cdot a$ by 2.
$\frac{20}{2}=\frac{2\cdot a}{2}$

Step2: Simplify the equation.

$10 = a$
However, there seems to be an error in the provided options as the correct answer $a = 10$ is not among them. If we assume there was a mis - typing in the original equation and we consider the equation as $20=- 2\cdot a$:

Step1: Isolate the variable a.

Divide both sides of the equation $20=-2\cdot a$ by - 2.
$\frac{20}{-2}=\frac{-2\cdot a}{-2}$

Step2: Simplify the equation.

$a=-10$

Answer:

a = - 10 (assuming the equation was meant to be $20=-2\cdot a$)