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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