1.

In a college 35% students are girls. Find the probability that: (i) two are girls, (ii) at least one is a girl in a random sample of 5 students.

Answer»

Let x be the number of girls is a binomial variate with the parameters n-5,p = 0.35 and
p = 1 -P
= 1 – 0.35 = 0.65.
Then the p is:
p(x) = ncpqn – x; x = 0,1,2, …… n
= 5cx (0.35)x (0.65)5-x; x = 0,1,2, …….. 5
(i) p( girls ) =p(x = 2) = = 5c2 (0.35)2 (0.65)5 – 2 

= 10 x 0.1225 x 0.2746 = 0.3364
(ii) p(at least one is a girl) = p(x > 1) 

= 1 – p (x < 1) = 1 – p (x = 0)
= 1 -5C(0.35)(0.65)5 – 0

= 1 – [1 x 1 x 0.1160] = 0.884



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