You may leave factorials and binomial coefficients unevaluat

You may leave factorials and binomial coefficients unevaluated in this question. A store sells 4 types of hard candies: cherry, strawberry, orange and lemon. How many ways are there to choose: (a) 20 candies with at least a piece of each flavor? (b) 20 candies with at least 4 cherry, 5 strawberry, 2 lemon and any amount (including zero) of orange?