Okay, here we have this:
Considering that we know that 10 ounces of popcorn cost $2.50, let's calculate how much does popcorn cost per ounce:
Cost of one ounce of popcorn=Cost of 10 ounces/10
Replacing we obtain:
Cost of one ounce of popcorn=$2.50/10
Cost of one ounce of popcorn=$0.25