To determine the cost of each bottle of juice (let's call it \( x \)), we first need to establish the total cost of the items Katherine bought.
- The cost of the bag of chips is $2.70.
- The total cost before tax is $15.78.
- She bought a 12-pack of juice bottles, so the total cost of the juice can be expressed as \( 12x \), where \( x \) is the cost of one bottle.
The equation that represents the total cost can be set up as follows:
\[ 2.70 + 12x = 15.78 \]
Now, we will solve for \( x \):
- Subtract 2.70 from both sides of the equation:
\[ 12x = 15.78 - 2.70 \]
Calculating the right side:
\[ 15.78 - 2.70 = 13.08 \]
So we have:
\[ 12x = 13.08 \]
- Now, divide both sides by 12:
\[ x = \frac{13.08}{12} \]
Calculating that gives:
\[ x = 1.09 \]
Thus, the cost of each bottle of juice is \( x = 1.09 \).
Answer: \( x = 1.09 \)