Combinations:

We have 21 balls in total. We have 7 different sizes of balls. Each ball is colored in 1 of 3 colors. (Blue, Pink, Black). In summary: there are 7 different sizes of pink balls, 7 different sizes of blue balls, 7 different sizes of black balls. All in total, we have 21 balls.

Compute the combinations and give a brief explanation your thinking process:

1. Select 3 balls. 2 black and 1 pink.

2. Select 3 balls. 2 black and 1 pink. No two picked balls can be the same size.

3. Select 7 balls, one of each size.