We know that one box of popcorn and one soda cost $5.10. We also know that two boxes of popcorn and one soda cost $8.35. To find how much a box of popcorn costs we can simply subtract $5.10 from $ 8.35. This will give us $3.25. Now we can subtract $3.25 from $5.10. This will give us $1.85. This means that a soda costs $1.85.