1.

If the probability of success is 0.09, how many trials are needed to have a probability of at least one success as 1/3 or more?

Answer»

Given p = 0.09 (success) 

q = 0.91 (failure) 

We have to find number of trials ‘n.’ 

According to the problem, 

P(X ≥ 1 ) > 1/3 

(We must have atleast one success) 

1 – P(X < 1) > 1/3 

1 – P(X = 0) > 1/3 

(or) P(X = 0) < 2/3 

Using p.m.f, we have, 

nC0(0.09) 0 (0.91) n < 2/3

(0.91)n < 2/3

we can use log tables to calculate (or) by trial method try for n = 1, 2,…… using calculator. 

We observe that (0.91)5 < 2/3 but (0.91)4 = 0.6857 > 2/3. Thus we need a minimum of 5 trials or more.



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