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The probability of a bomb hitting a bridge is 1/2 and two direct hits are needed to destroy it. The least number of bombs required so that the probability of the bridge being destroyed is greater than 0.9 is:
  • a)
    7 bombs
  • b)
    3 bombs
  • c)
    8 bombs
  • d)
    9 bombs
Correct answer is option 'A'. Can you explain this answer?
Verified Answer
The probability of a bomb hitting a bridge is 1/2 and two direct hits ...
Try to find the number of ways in which 0 or 1 bomb hits the bridge if n bombs are thrown.
The required value of the number of bombs will be such that the probability of 0 or 1 bomb hitting the bridge should be less than 0.1.
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Most Upvoted Answer
The probability of a bomb hitting a bridge is 1/2 and two direct hits ...
Solution:

Given: Probability of a bomb hitting a bridge = 1/2 and two direct hits are needed to destroy it.

Let's calculate the probability of the bridge not being destroyed after n bombs are dropped.

Probability of not destroying in the first trial = 1/2
Probability of not destroying in the second trial = 1/2
Probability of not destroying in the third trial = (1/2) * (1/2) = (1/2)^2
Probability of not destroying in the fourth trial = (1/2)^3
.
.
.
Probability of not destroying in the nth trial = (1/2)^(n-1)

So, the probability of the bridge being destroyed after n trials is given by:

P(Bridge destroyed after n trials) = 1 - (1/2)^(n-1)

We need to find the least number of bombs required so that the probability of the bridge being destroyed is greater than 0.9.

i.e., P(Bridge destroyed after n trials) > 0.9

1 - (1/2)^(n-1) > 0.9
(1/2)^(n-1) < />
n-1 > log2(0.1)
n > log2(0.1) + 1
n > 6.32

So, the least number of bombs required to destroy the bridge with a probability greater than 0.9 is 7 bombs.

Hence, option A is the correct answer.
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