Hey!
First, let's write the problem.
[tex]12=0.9a[/tex]
Let's switch sides. This is done so that we end up with the [tex]a[/tex] on the left side.
[tex]0.9a=12[/tex]
To remove the decimal, let's multiply both sides by 10.
[tex]0.9a\cdot \:10=12\cdot \:10[/tex]
[tex]9a=120[/tex]
Divide both sides by 9.
[tex]\frac{9a}{9}=\frac{120}{9}[/tex]
This tells us that our final answer would be,
[tex]a=\frac{40}{3}[/tex]
Thanks!
-TetraFish