The 20th expression would be 80. Counted on my fingers xD hit 80 when i did 20 fingers. which means that if you put 20 as n, it should come out to 80, but it comes out to 84 by the answer you chose, so I'm wondering if the answer you chose was right, or if it was A
Answer:
0.0782 (7.82%)
Step-by-step explanation:
The required probability will be found by using the Binomial Distribution which fits the case of n independent events each with a probability of success equal to p with k successes.
The PMF (Probability Mass Function) is

Where 
LabTech states that the probability that a microscope is defective is 0.17%, p=0.0017, q=0.9983. We need to know the probability that k=1 microscope is defective out of a set of n=50 of them. We now apply the formula

Which means that there is a 7.82% of probability to get 1 defective microscope out of the first 50
-5a+3b should be the answer simplified