Answer: Let's call the bill before the tip "x".
The tip was 20.7% of the bill, so the amount of the tip can be expressed as:
tip = 0.207 * x
The total bill, including the tip, was $31.09, so we can write an equation to represent this:
x + tip = 31.09
Substituting the expression for the tip into this equation, we get:
x + 0.207x = 31.09
Expanding and solving for x, we get:
1.207x = 31.09
x = 31.09 / 1.207
x = 25.76
So the bill before adding the tip was $25.76.
Explanation: