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Anna and Mark

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Junior | Next Rank: 30 Posts
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Anna and Mark

by imago » Tue Sep 18, 2007 1:27 pm
Anna and Mark are included in a group of 8 people. If 4 people are selected at random and without replacement from this group, what is the probability that Mark and not Anna will be selected?

a.1/7
b.2/7
c.3/7
d.4/7
e.5/7


OA: b
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by kajcha » Tue Sep 18, 2007 2:01 pm
It is B

Suppose the combination of people selected is Mxxx

where x represents any person other than Anna

Probability of this combination = (1/8)*(6/7)*(5/6)*(4/5) = 1/14

Now Mark can be selected at 2nd, 3rd or 4th position too. so,

total probability = 4*1/14 = 2/7
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