A vaccine is 95 percent effective. What is the probability that it is not effective for more than 1 out of 20 individuals?

The probability that the vaccine is not effective in more than 1 of the 20 individuals is equal to:

P = 1 - P(1) - P(0)

Where:

P(1) = probability that the vaccine is not effective in one individual

P(0) = probability that the vaccine is not effective in 0 individuals.

The probability that the vaccine is effective is 0.95

Then the probability that the vaccine is not effective is 0.05.

Then to calculate P(1), suppose that the vaccine is not effective in the first case and then is effective 19 times, this is:

p(1) = (0.05^1)*(0.95^19)

But this only for one case, we have 20 possible permutations (the vaccine is not effective in the first case, the second case, third case, etc)

Then the actual probability is:

P(1) = 20*((0.05^1)*(0.95^19)) = 0.377

And P(0) is easy to compute:

P(0) = (0.05^0)*(0.95^20) = 0.358

And here we have only one possible permutation, so we do not worry about that.

Then P = 1 - P(1) - P(0) = 1 - 0.377 - 0.358 = 0.265

This means that the probability that the vaccine is not effective in more than one individual is of 0.265 (or 26.5% in percentage form)

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