A natural number is chosen at random from amongst the first 300. What is the probability that the number chosen is a multiple of 2 or 3 or 5 ? -Maths 9th

1 Answer

Answer :

(b) \(rac{11}{15}\)n(S) = 300 Let A : Event of getting a number divisible by 2 B : Event of getting a number divisible by 3 C : Event of getting a number divisible by 5 ∴ A ∩ B : Event of getting a number divisible by both 2 and 3, i.e., 6 B ∩ C : Event of getting a number divisible by both 3 and 5, i.e, 15 A ∩ C : Event of getting a number divisible by both 2 and 5, i.e., 10 A ∩ B ∩ C : Event of getting a number divisible by all 2, 3 and 5, i.e., 30 Then, n(A) = 150, n(B) = 100, n(C) = 60 n(A ∩ B) = 50, n(B ∩ C) = 20, n(A ∩ C) =30, n(A ∩ B ∩ C) = 10 ⇒ P(A) = \(rac{150}{300},\) P(B) = \(rac{100}{300},\)  P(A) = \(rac{60}{300}\)P(A ∩ B) = \(rac{50}{300},\) P(B ∩ C) = \(rac{20}{300},\) P(A ∩ C) = \(rac{30}{300},\) P(A ∩ B ∩ C) = \(rac{10}{300}\)∴ P(Number divisible by 2 or 3 or 5) = P(A ∪ B ∪ C) = P(A) + P(B) + P(C) – (P(A ∩ B) – P(B ∩ C) – P(A ∩ C)) + P(A ∩ B ∩ C)= \(rac{150}{300}\) + \(rac{100}{300}\) + \(rac{60}{300}\) - \(\bigg(\)\(rac{50}{300}\) + \(rac{20}{300}\) + \(rac{30}{300}\)\(\bigg)\) + \(rac{10}{300}\)= \(rac{320}{300}\) - \(rac{100}{300}\) = \(rac{220}{300}\) = \(rac{11}{15}\).

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