Probability?? (1 Viewer)

[Premus]

hey

the numbers 1,2,3.....9 are written on 9 cards. 3 cards are chosen at random.

1) find the probability that the sum of the 3 numbers chosen is equal to 9.
2) if it is known that the first number chosen is 2, find the probability now that the sum of the 3 numbers chosen is equal to 9.

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a test contains of 7 multiple choice questions all of which are to be attempted. each question contains 4 alternative answers of which one is correct. find the number of ways in which 7 questions can be attempted so that exactly 2 questions are answered correctly.

Thanks a lot!!

 

[Affinity]

126
135
234

so it's 3/(9 C 3 )

2 + 16
2 + 34

so it's 2/(8 C 2)

 

[xiao1985]

lolz a bit later than affinity >< but oh well, used a dumber method too =(

man, i am losin grip on prob/perm/comb