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fallenshadow
May 20th, 2012, 04:23 PM
Hi guys/gals,

Recently I have found that I have forgotten how to do simple enough maths problems relating to probablity and statistics.

I would appreciate if someone could help me out.


Machines A, B, C produce respectively 20, 30 and 50 percent of the total output. Of their respective outputs 5, 3 and 2 percent are defective. An item is chosen at random.

1. What is the probablity that it is defective?

2. If the item is defective, what is the probability it was produced by machine A?

Does this need a formula or can these be figured out with simple logical math?

zombifier25
May 20th, 2012, 04:48 PM
(shoot me if I'm wrong)
I think the answer for 1 is 20*5/100 + 30*3/100 + 50*2/100, which equals 2.9%. And for 2, it's 1/2.9*100 which roughly equals 34.5%

fallenshadow
May 20th, 2012, 05:15 PM
Yes I think you are right. I was pretty sure that you can do this without a special formula.

Another one that has me extremely puzzled is this one:


A batch that has 2 or more defectives are found in a random sample of 100 is rejected.

1. Probability of accepting a batch that is 1% defective?

2. Probability of rejecting a batch that is 5% defective?



Obviously the difference in defective percent doesn't matter too much but I don't know what to do for the difference between being "accepted" or "rejected". :?

fallenshadow
May 21st, 2012, 03:23 PM
And for 2, it's 1/2.9*100 which roughly equals 34.5%

Actually I think this part may be wrong. You multiplied the overall failure rate when it should be the failure rate of A.

(of course correct me if I am wrong)

zombifier25
May 21st, 2012, 03:25 PM
Yes I think you are right. I was pretty sure that you can do this without a special formula.

Another one that has me extremely puzzled is this one:



Obviously the difference in defective percent doesn't matter too much but I don't know what to do for the difference between being "accepted" or "rejected". :?

The size of the batch is random or what?

fallenshadow
May 22nd, 2012, 03:07 PM
Yes it is a random sized batch.