Let x represent the amount it would cost you to buy 7 cans of fruit cocktail.
We were told that the store sells 8 cans of fruit cocktail for $20. This means that
If 8 = 20
then 7 = x
By cross multiplying, it becomes
8x = 20 * 7
8x = 140
x = 140/8
x = $17.5
We can also solve it by finding the unit rate
If 8 costs $20,
then 1 would cost 20/8 = $2.5
Therefore, the cost of 7 would be
7 * 2.5 = $17.5