Question

3. Two balls are drawn in succession without replacement from an urn containing 4 red balls and 3 blue balls. Find the probability mass function (P.M.F.) for the number of red balls draw.

Answer 1

X ____ 0 ______ 1 _______ 2

P(x) __ 1/7 _____ 4/7 _____ 2/7

Explanation:

Given that :

Number of red balls = 4

Number of Blue balls = 3

Total number of balls = (4 + 3) = 7

Picking without replacement :

Number of picks = 2

1st pick is red :

Probability = (required outcome / Total possible outcomes)

USing combination formula :

nCr = n! / (n-r)! r!

Total possible outcome :

7C2 = 7! / 5!2! = 7*6 / 2 = 42 / 2 = 21

X = number of red

For red = 0;

P(x =0) = required outcome / Total possible outcomes

P(x = 0)

Required outcome : [4C0 * 3C2)]

P(x = 0) = [(4C0 * 3C2)] / 21 = (1*3)/21 = 3/21 = 1/7

P(x = 1)

Required outcome : [4C1 * 3C1)]

P(x = 1) = [(4C1 * 3C1)] / 21 = (4*3)/21 = 12/21 = 4/7

P(x = 2)

Required outcome : [4C2 * 3C0)]

P(x = 2) = [(4C2 * 3C0)] / 21 = (6*1)/21 = 6/21 = 2/7

Hence ;

X ____ 0 ______ 1 _______ 2

P(x) __ 1/7 _____ 4/7 _____ 2/7