lsquo A rsquo speaks truth in 75 of the cases and lsquo B rsquo in 80 of the cases In what per...........

Quantitative Aptitude ( CCS) Permutation & Combination

‘A’ speaks truth in 75% of the cases and ‘B’ in 80% of the cases. In what percentage of cases are they likely to contradict each other, narrating the same incident?

15%

35%

45%

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Explanation:

Let A = Event that A speaks the truth
B = Event that B speaks the truth

Then P(A) = 75/100 = 3/4
P(B) = 80/100 = 4/5

P(A-lie) = 1 - 3/4 = 1/4
P(B-lie) = 1 - 4/5 = 1/5

Now
A and B contradict each other =
[A lies and B true] OR [B true and B lies]
= P(A).P(B-lie) + P(A-lie).P(B)
[Please note that we are adding '+' at the place of OR]
= (3/5*1/5) + (1/4*4/5) = 7/20
= (7/20 * 100) % = 35%