Let \( x \) represent the original bill amount. The 12.00 dessert was removed from this bill, so the reduced bill is:
\[ x - 12.00 \]
Stan tipped 20% on the reduced bill, which can be expressed as:
\[ 0.20 \times (x - 12.00) \]
According to the problem, this tip amounts to 15.00. Therefore, we can write the equation:
\[ 0.20 \times (x - 12.00) = 15.00 \]
This is the equation that will determine the amount of the original bill \( x \).