Out of 6 black, 4 red, 2 white and 3 blue marbles in a box, find the probability of choosing at least 1 red marble when 4 marbles are randomly picked.

Choosing 4 marbles means 1^st marble AND then 2^nd AND then 3^rd AND then 4^th marble
At least 1 red marble means there can be 1, 2, 3, or 4 red marbles.
So, Probability of choosing red = 1 - Probability of not choosing red
If we remove red marbles, then 15 - 4 red marbles = 11 marbles remain.
We have to choose 4 out of these 11.

So Probability of choosing 4 marbles that are not red = 11/15 × 10/14 × 9/13 × 8/12 = 22/91.

∴ Probability of picking at least 1 red marble = 1 - 22/91 = 69/91.
[Here we reduce the denominator i.e. the total number of marbles because once we remove a marble from the box we do not put it back in the box.
So while removing the 2^nd marble, there are only 15 - 1 = 14 marbles in the box. The same is for the 3^rd and 4^th marbles.]