Determine the cost of 2 cereal box.
So if individual 2 cereal box of $5.18 then one cereal box is free means that individual buy 3 cereal box for $5.18.
If individual save $1 per box then cost of one cereal box is $1.59.
Determine the price of three box with each cost box cost is $1.59.
Thus save $1 per box is better incentive as individual pay less for same number of boxes.