Mr Jacobs placed his folders on three chairs in a conference room comprising of 12 chairs, if Mrs Roberts walked in and also placed her folders on 3 chairs, what is the probability that she did no place any of her folders on a chair already containing Mr Jacobs folder?.	Choose one.
A) 3/9  B) 3/55  C) 9/12  D) 21/55  E) 3/12
Since Mr Jacob's folders are already on 3 chairs, the subset from which the 3 chairs on which Mrs Roberts placed her folders should be chosen are the 9 chairs not occupied by Mr Jacob's folders from the universe of 12.   The first chair can be one of the 9 from the 12 with probability 9/12.   The second chair can be one of the 8 from the 11 remaining with probability 8/11.   The third chair can be one of the 7 from the 10 remaining with probability 7/10.   The total probability will be 9/12 × 8/11 × 7/10. On cancellation, this comes to 21/55.   Thus, the answer is 21/55 - 21/55
<result>D</result>
Mr Jacobs placed his folders on three chairs in a conference room comprising of 12 chairs, if Mrs Roberts walked in and also placed her folders on 3 chairs, what is the probability that she did no place any of her folders on a chair already containing Mr Jacobs folder?.