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Q.

The probability that a missile hits a target successfully is 0.75. In order to destroy the target completely, at least three successful hits are required. Then the minimum number of missiles that have to be fired so that the probability of completely destroying the target is NOT less than 0.95, is _____

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answer is 6.

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Detailed Solution

Let P(r)= probability of r successes =nCr34T14n−r1−(P(0)+P(1)+P(2))≥0.95⇒ 1−nC014n−nC13414n−1−nC234214n−2≥0.95⇒ 1−1+3n+9n(n−1)24n≥0.95⇒ 9n2−3n+2≤0.05×4n×2≤4n10 for n=5 212≤102.4  (Not true)  for n=6 308≤409.6  true ∴  least value of n=6
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The probability that a missile hits a target successfully is 0.75. In order to destroy the target completely, at least three successful hits are required. Then the minimum number of missiles that have to be fired so that the probability of completely destroying the target is NOT less than 0.95, is _____