## Book: RD Sharma - Mathematics (Volume 2)

### Chapter: 33. Binomial Distribution

#### Subject: Maths - Class 12th

##### Q. No. 36 of Exercise 33.1

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36
##### It is known that 60% of mice inoculated with serum are protected from a certain disease. If 5 mice are inoculated, find the probability thati. none contract the diseaseii. more than 3 contract the disease.

Given that, the probability that the mice are protected from a certain disease is 60%.

Also, the sample size of mice = 5

Let p be the probability of the mice not contracting with a certain disease.

Then, q is the probability of the mice contracting with a certain disease.

p = 60%

And we know that,

p + q = 1

q = 1 – p

Let X be a random variable representing a number of mice contracting with the disease.

Then, the probability of r mice contracting with the disease out of n mice inoculated is given by the following binomial distribution.

P (X = r) = nCrqrpn-r

Putting the values,

n = 5,

&

We get

…(A)

(i). We need to find the probability that none of the mice contract with the disease.

For this, put r = 0.

We get the probability as,

Probability = P (X = 0)

From equation (A),

P (X = 0) = 0.07776

, the probability that none of the mice contracts with the disease is 0.07776.

(ii). We need to find the probability that more than 3 mice contract the disease.

So, probability = P (X = 4) + P (X = 5)

Probability = 0.2125

, the probability that more than 3 mice contract the disease is 0.2125.

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