$12 is the price of the shirt after 20% discount.
So, if we let the original price of the shirt be x, we can say:
What number (x), decreased by 20% gives us 12??
Or,
x - 20% of x = 12
We need to convert the percentage to a decimal by dividing by 100. So, 20/100 = 0.2
Now, we can write
[tex]x-0.2x=12[/tex]With a little algebra, we can solve for x, the original price. Shown below:
[tex]\begin{gathered} x-0.2x=12 \\ 0.8x=12 \\ x=\frac{12}{0.8} \\ x=15 \end{gathered}[/tex]The original price of the shirt
$15