We can assume a probability distribution for this kind of problem. We can say that the number of defective pages is a random variable X. The probability of a page being defective is random and = 100/1000 = 0.1

In the first 50 pages, the expected number of defective pages is 50 * 0.1 = 5. Let X denote the number of defective pages in the first 50 pages.

We can have 0 defective pages, 1 or 2 or 5 or 10 or 19 or 20 or up to 50 defective pages in the first 50 pages of the book. That means,

0 <= X <= 50 and E(X) = λ = mean or expected value = 5

We can assume the Poission probability distribution or the Normal probability distribution to solve this. We assume that X follows the Possion's probability distribution function.

Let us calculate the probabilities, with λ = 5.

P(X=10) <= 0.0181 = the probability that the number of defective pages is 10.

P(X>= 10) = 0.145. This is the probability that the number of defective pages is equal to or more than 10.

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for info:

we can also calculate P(X=0) = 0.006738

P(X=1)=0.0337 P(X=5) = 0.175

P(X=8) = 0.0653 P(X=9) = 0.0362