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Probability Old Exam P Forum 

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#1




One problem in SOA study manual
Hi everybody, I am confused by this problem:
I think, in terms of calculating the probability, we should not use combination, rather, we should use permutation, because the order actually matters (picking QND counts differently from picking DNQ), so my solution would be total: 6*5*4 = 120 one Q: 4*3 = 12 two Q: 4 probability would be (12*2+4)/120 what is wrong with my thoughts? thanks~ 
#2




You need to explain yourself. Why does order matter, and where did your calculations except 6*5*4 come from?
You could use permutations, with 120 as the denominator, as long as you use the correct corresponding numerator. There is no need or advantage to using permutations. 
#3




Quote:

#4




If you want to use order, write down all the possible orders that work, and then calculate how many ways there are of getting that order. You would then conclude that the combination method works.
For example, Qnd works, and there are 2*3*1 ways of getting that order. 
#5




For this particular problem the easiest way is to calculate the probability of the complement which is the probability of not picking a quarter. You could do this by multiplying probabilities (4/6)(3/5)(2/4) or by using a counting rule.

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