If seven persons sit in a row, then the probability that three particular persons are always together is? Choose one.
A) 1/5 B) 1/4 C) 1/9 D) 1/6 E) 1/7
Seven persons can be arranged in a row in 7! ways.   Treat the three persons to sit together as one unit then there five persons and they can be arranged in 5! ways.   Again three persons can be arranged among them selves in 3! ways.   Favourable outcomes = 3!5!   Required probability = 3!5!/7! = 1/7   Answer:1/7
<result>E</result>
If seven persons sit in a row, then the probability that three particular persons are always together is?