Answer:
Original retail price: $35
Explanation:
20% off means that the discounted price is equivalent to 80% (100% - 20%) of the retail price. 
Calling x the retail price of the scuba mask and snorkel set, then a discounted price of $28 means that this amount of money is 80% of x. Mathematically: 
80*x/100 = $28 
x = $28*100/80 
x = $35