If it is known that the probability that a light bulb works correctly

jippie771h

jippie771h

Answered question

2021-11-17

If it is known that the probability that a light bulb works correctly is 9 times greater than the probability that it has a fault, What is the maximum number of light bulbs they must make to have a probability greater than 0.5 that all of them work correctly?

Answer & Explanation

soniarus7x

soniarus7x

Beginner2021-11-18Added 17 answers

Step 1
Binomial distribution is discrete distribution that deals with the probabilities of success and failure. In a binomial experiment there should only be exactly 2 outcomes, success and failure.
The probability of success should be same across all the trials. The probability mass function of binomial distribution is given below.
P(X=x)=(nx)px(1p)nx
Step 2
Let p be the probability that a bulb has fault. Then 9p is the probability that the bulb work correctly. The total probability is always 1. So,
p+9p=1
10p=1
p=110
=0.1
So 9p=0.9. Thus 0.9 is the probability that the bulb work correctly. Let the binomial random variable X indicates the number of correctly working bulbs in n number of bulbs. So P(X=n) indicates the probability that all of the n bulbs works correctly. It is said that this probability should be greater than 0.5. So,
P(X=n)>0.5
(nn)0.9n(10.9)nn>0.5
0.9n>0.5
0.9n>0.5
Now solve 0.9n>0.5 for n. First find the critical point of the inequality.
0.9n=0.5
log(0.9n)=log(0.5)
nlog(0.9)=log(0.5)
n=log(0.5)log(0.9)
6.58
Now check the critical value with the inequality 0.9n>0.5. The inequality holds if n<6.57. n can only take whole numbers. So maximum number of bulbs must make to have a probability greater than 0.5 is 6.

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