# Thread: 20% of memory chips.....

1. Originally Posted by zorro
Question :

20% of memory chips made in certain plant are defective. Find the probability that in a lot of 100 randomly chosen for inspection.
i) atmost 15 will be defective
ii) exactly 15 will be defective
Obtain these probability also by using Normal Approximation to binomial probabilities.
The number of defectives in the batch has a binomial distribution $B(100,0.2)$ (no approximations at this point).

The approximation you are asked to use is the normal approximation to the binomial. Where $X \sim B(N,p)$ is approximated as $X\sim N(Np,Np(1-p))$.

CB

2. ## U r confusing me

Originally Posted by CaptainBlack
The number of defectives in the batch has a binomial distribution $B(100,0.2)$ (no approximations at this point).

The approximation you are asked to use is the normal approximation to the binomial. Where $X \sim B(N,p)$ is approximated as $X\sim N(Np,Np(1-p))$.

CB

NOw is this correct

$X\sim N(Np,Np(1-p))$= N(20,16).........Is it correct now?

3. Originally Posted by zorro
NOw is this correct

$X\sim N(Np,Np(1-p))$= N(20,16).........Is it correct now?
Yes. Now your job is to use this distribution to calculate the required probabilities (don't forget to use the continuity correction in your calculations).

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### If 20% of the memory chips made in a certain plant are defective, what are the probabilities that in a lot of 100 random chosen for inspection? a. at most 15.5 will be defective b. exactly 15 will be defective Hint: calculate it in binomial dist. And norm

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