First you divide $2.80 by 20 in order find out how much one ounce of popcorn costs. This gives you $0.14 per ounce of popcorn. You then multiply this number by 45 to get the cost of a 45 ounce bag of popcorn.
2.80 / 20 = .14
.14 * 45 = 6.3
A 45 ounce bag of popcorn costs $6.30