The equation that represents the situation when Sherry and Damien will have the same amount of money in their cash boxes is 18 + 0.5x = 12 + 0.75x
Solution:
Given that
Initial amount of sherry in cash box = $18
Sherry is selling bags of chips.
Price of 1 bag of chips = 50 cents = $ 0.5
Initial amount of Damien in cash box = $12
Damien is selling ice cream sandwich .
Price of 1 ice cream sandwich = 75 cents = $ 0.75
Lets create expression of amount in cash box for sherry and daimen when "x" items are sold.
In case of Sherry price of "x" bags of chips

Amount in Sherry cash box after "x" items are sold = initial amount + price of "x" bags of chips
Amount in Sherry cash box after "x" items are sold = 18 + 0.5x ------ (1)
In case of Damien price of "x" ice cream sandwich =

Amount in Damien cash box after "x" items are sold = initial amount + price of "x" ice cream sandwich
Amount in Damien cash box after "x" items are sold = 12 + 0.75x ------ (2)
As we required expression when amount in both the cash boxes must be equal, on equating (1) and (2) we get
18 + 0.5x = 12 + 0.75x
Hence expression 18 + 0.5x = 12 + 0.75x represents a condition when amount in both the cash boxes will be same.