Abag of candy contains 300 pieces of which 28% are red.
how many pieces are not red

300 = Total candies
28% = Percent of candies that are red

100% - 28% = 72% 
72% = Percent of candies that are NOT red

My personal preference for setting up this type of question:

300 -> 100% (Total candies)
___ -> 72% (NOT red candies)
___ -> 28% (Red candies)

Next:

28% = 0.28
300 x 0.28 = 84
84 = Number of candies that are red

300 -> 100% (Total candies)
___ -> 72% (NOT red candies)
84   -> 28% (Red candies)

Now:

300 - 84 = 216

300 -> 100% (Total candies)
216 -> 72% (NOT red candies)
84 -> 28% (Red candies)

216 equals the number of candies that are NOT red.