permutation and combination

i)let's divide the balls into white and non white categories.

So, there are

6 NW balls and 1W ball.

If we must include the white ball,in the selection of 4 balls in total, we have to select 3 balls from the NW category and we are done.

So the answer is to select 3 from 6

ii) to exclude the red and black  ball ,we must select 4 balls from the remaining 5 balls

iii)to include blue, but not green is the same as excluding green ball.

So select 4 balls from the remaining 6 balls.

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