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## Homework Statement

A quality control engineer tests the quality of produced computers. Suppose that 5% of computers have defects, and defects occur independently of each other.

A- What is the expected number of defective computers in a shipment of twenty?

B- Find the probability of exactly 3 defective computers in a shipment of twenty.

C- Find the probability that the engineer has to test at least 5 computers in order to find a defective one.

## Homework Equations

## The Attempt at a Solution

A-1

B- .061 which were correct

For C I got .00257 which was wrong. I just don't understand how I'm supposed to find at least 5 when I don't know the total number since for finding .00257 I used n=20 which I guess I wasn't supposed to since I got it wrong