suganyam wrote:
hi can some kindly point out my mstake
3c2/12c2+(3c1*9c1/12c2)2
I understand that it should be 3c1/12*9c1/11 but how should i distinguish them. like which one to use where Kindly guide me
suganyam wrote:
hi can some kindly point out my mistake
3c2/12c2+(3c1*9c1/12c2)2
I understand that it should be 3c1/12*9c1/11 but how should i distinguish them. like which one to use where Kindly guide me
Zillion thanks in advance
Question: In a box of 12 pens, a total of 3 are defective. If a customer buys 2 pens selected at random from the box, what is the probability that neither pen will be defective?
There are 9 non-defective pens out of 12
If you want to pick simultaneously VS you pick one at a time
without replacement, the answers would be
SAMEIf you want to do this by picking simultaneously, you should interpret the question as:
Favorable: Pick 2 pens that are not defective i.e. pick 2 pens from 9 = 9C2 ways = 9*8/2! = 36 ways
Total: Pick 2 pens from 12 = 12C2 ways = 12*11/2! = 66 ways
=> Probability = 36/66 = 6/11
If you want to do this by picking one at a time, you should interpret the question as:
Pick 1st pen which is not defective AND Pick 2nd pen which is not defective (Note: AND means MULTIPLY)
Probability of picking 1st pen which is not defective = 9/12
Not, since we pick WITHOUT REPLACEMENT, we have 8 non-defective out of 11
Thus, probability of picking 2nd pen which is not defective = 8/11
Thus, required probability = 9/12 * 8/11 = 6/11
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