maths
posted by Anonymous .
urn contains 4 white and 6 red rolls. Four balls are drawn at random (without replacement) from the urn. Find the probability distribution of number of white balls?

prob(white) = 4/10 = 2/5, prob(red) = 6/10 = 3/5
no white balls  (3/5)^4 = 81/625
one white ball  C(4,1) (2/5) (3/5)^3 = 216/625
two white balls  C(4,2) (2/5)^2 (3/5)^2 = 216/625
three white balls  C(4,3) (2/5)^3 (3/5) = 96/625
four white balls  (2/5)^4 = 16/625
notice that the sum of these is 1 , as it should be