Math
posted by HELP HELP HELP MNHS .
In a candy shop, the shopkeeper allows the exchange of 3 sweet rappers for 1 new sweet. John has 81 sweets at first. What is the largest possible number of sweets that john could have eaten?
(with solution pls... tnx...)

81 wrappers gets 27 more
27 wrappers gets 9 more
9 wrappers gets 3 more
3 wrappers gets 1 more
81+27+9+3+1 = 121