Probability 2 Question 13

13. In a certain city only two newspapers A and B are published, it is known that 25 of the city population reads A and 20 reads B, while 8 reads both A and B. It is also known that 30 of those who read A but not B look into advertisements and 40 of those who read B but not A look into advertisements while 50 of those who read both A and B look into advertisements. What is the percentage of the population reads an advertisement?

(1984, 4M)

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Answer:

Correct Answer: 13. False

Solution:

  1. Let P(A) and P(B) denote respectively the percentage of city population that reads newspapers A and B.

Then,

P(A)=25100=14,P(B)=20100=15,P(AB)=8100=225,

P(AB)=P(A)P(AB)=14225=17100,

P(A¯B)=P(B)P(AB)=15225=325

Let P(C) be the probability that the population who reads advertisements.

P(C)=30 of P(AB¯)+40 of P(A¯B)

+50 of P(AB)

[since, AB¯,A¯B and AB are all mutually exclusive]

P(C)=310×17100+25×325+12×225=1391000=13.9



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