To find out how many sodas Mark bought, we can set up the following equation based on the information given:
- Mark spent $78 on movie tickets.
- The total amount spent on tickets and sodas is $103.50.
- Each soda costs $4.25.
Let \( S \) be the number of sodas that Mark bought. We can express the total cost of the sodas as \( 4.25 \times S \).
The total cost equation can be written as: \[ 78 + 4.25S = 103.50 \]
Now, we will solve for \( S \):
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Subtract the cost of the tickets from the total: \[ 4.25S = 103.50 - 78 \] \[ 4.25S = 25.50 \]
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Now, divide by the cost of each soda: \[ S = \frac{25.50}{4.25} \] \[ S = 6 \]
Therefore, Mark bought 6 sodas.
So the answer is: 6.