Chapter 6 - Section 6.5 - Generalized Permutations and Combinations - Exercises - Page 432: 8

225792840 ways

The order of selecting donuts do not matter , and we know that ways to select r unordered elements from a set with n elements with repetition is given by $C$(n+r-1,r) Here we need to select 12 donuts from 21 varieties of donuts available Hence number of ways is given by $C$(21+12-1,12) = $C$(32,12) = 225792840 ways