if we toss 4 coins and we want to know the probability of having 3 heads we can easily calculate that using pascal triangle . I want to know if we have an Urn that contain 3 balls : 1 black , 1 red, 1 yellow. and we want to draw 3 balls , but when we draw the first ball we put it back in the urn and thn we draw the 2nd ball which thn we put it back and finally we draw a third ball. in this example we have 27 possible way of selecting 3 balls. I want to know how can we calculate the probability of having at least 2 black ball , without using a tree diagram, probably wiht pascal triangle or any other formula with the explanation. and you are very thankfull. It is not a homework , I just want to know how this kind of problems works.