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**Solve the following problem :**

In a large school, 80% of the students like mathematics. A visitor asks each of 4 students, selected at random, whether they like mathematics.

Find the probability that the visitor obtains the answer yes from at least 3 students.

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#### Solution

Let X denote the number of pupils who like mathematics.

P(pupils like mathematics) = p = `(8)/(100) = (4)/(5)` ...[Given]

q = 1 – p = `1 - (4)/(5) = (1)/(5)`

Given, n = 4

∴ X ~ B`(4, 4/5)`

The p.m.f. of X is given by

P(X = x) = `""^4"C"x 4/5^x (1/5)^(4 - x), x` = 0, 1, ...,4

P(the visitor obtains the answer yes from at least 3 students)

= P(X ≥ 3)

= P(X = 3 or X = 4)

= P(X = 3) + P(X = 4)

= `""^4"C"_3 (4/5)^3 (1/5)^1 + (256)/(5^4)` ...[From (i)]

= `(4^4)/(5^4) + (256)/(5^4)`

= `(256)/(5^4) + (256)/(5^4)`

= `(512)/(5^4)`.