Self Doubt on Combinatory Discrete Mathematics

Question

Given there are 3 full baskets of apples, mangoes, and oranges. How many ways possible if

a) You need to buy any 4 fruits out of these 3 baskets ?

b) you buy any 4 fruits such that you take at least one from each basket ?

Answer 1

Let number of Apples, Mangoes, Oranges bought be $N_A, N_M,N_O $

1. This is number of possible non negative integral solutions of $N_A+N_M+N_O=4$
2. If we pick one one apple, one mango and one orange, we are left with 3 choices to pick up the 4^th fruit. In other words, this is number of possible non negative integral solutions of $N_A+N_M+N_O=1$

This is based on the assumption that there are atleast 4 apples, mangoes and oranges in the baskets.