variance and standard deviation?

Question

A production system with five production lines produces thousands of items per day. From past experience it is known that one out of every 10 products coming out of each production line is defective. The probability of exactly two defective items in a sample of 12 is what?

Answer 1

₁₂C₂*(1/10)²*(9/10)¹⁰

 

The formula above broken down:

₁₂C₂ -- this is the number of ways you can choose a sample of 12 and have 2 of them be defective.

(1/10)²-- this is the probability of selecting a defective item, raised to the number of items you intend to be defective.

(9/10)¹⁰-- this is the probability of selecting a non-defective item, raised to the number of items you intend to be non-defective. 

 

You must multiply all three parts to find the probability.