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distinct numbers

by shashank.ism » Sat Feb 20, 2010 1:39 pm
There are nine distinct numbers of which five numbers are positive and four numbers are negative. Three numbers are chosen at random and the product of these numbers is found. How many of these products are positive?

A) 48
B) 300
C) 40
D) 90
E) None of these

CA is C
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by ajith » Sat Feb 20, 2010 8:58 pm
shashank.ism wrote:There are nine distinct numbers of which five numbers are positive and four numbers are negative. Three numbers are chosen at random and the product of these numbers is found. How many of these products are positive?

A) 48
B) 300
C) 40
D) 90
E) None of these

CA is C
The product is positive if

1) Two numbers are negative and one positive - select 2 negative num are 1 positive - 4C2*5C1 = 6*5 =30
2) All the three numbers are positive - 5C3 = 5*4/2 = 10

Total no of ways = 30+10 =40
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