Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

The question is this:

A shipment contains $400$ boxes of components. A shipment is returned if $> 90$ boxes are rejected. History shows that $20\%$ of boxes are usually rejected. What is the probability of a shipment being returned?

I tried to use normal approximation; however I don't have standard deviation or sample deviation. How can I solve this?

share|cite|improve this question
I presume that each component box is rejected independently w.p. $20\% = .2$. Then the number of rejected components is a binomial $\mathrm{Bin}(400, .2)$. Can you go from here? – Srivatsan Nov 26 '11 at 21:06

The standard deviation of a binomial is $\sqrt{n p (1-p)}$. Use this along with the normal approximation to solve your problem.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.