A bag contains 6 red balls and 5 black balls . Two balls are picked , one after the other . Find the probability that : i. Both are black
ii. The first is black
iii. Both are of the same colour
Given that :
a. it is without replacement
b. it is with replacement

There are:

6 red balls - R5 black balls - BTotal number = 11 ballsA. Without replacement

i. Two blacks

P(BB) = 5/11*4/10 = 2/11

ii. The first is black

P(B) = 5/11

or, alternatively

P(BR or BB) = 5/11*6/10 + 2/11 = 3/11 + 2/11 = 5/11

iii. Both are of same colour

P(BB or RR) = 2/11 + 6/11*5/10 = 2/11 + 3/11 = 5/11B. With replacement

i. Two blacks

P(BB) = 5/11*5/11 = 25/121

ii. The first is black

P(B) = 5/11

or alternatively

P(BR or BB) = 5/11*6/11 + 25/121 = 30/121 + 25/121 = 55/121 = 5/11

iii. Both are of same colour

P(BB or RR) = 5/11*5/11 + 6/11*6/11 = 25/121 + 36/121 = 61/121

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