Two trains start at the same lime from two stations A and B and proceed towards each other at speeds of 30 kmh-' and 25 kmlr1 respectively. When they meet, the difference between the distances travelled by these two trains is 40 km. What is the distance between A and B ?
without any restriction, 11 persons can sit in 11! ways, If we take all the girls together as one, the total number of persons will be 7 + 1 = 8, who can sit in 8! ways and the four girls can sit in 4! ways among themselves. So, when all the girls sit together the number of ways = 8! x 4!
When they don't sit together, the total number of ways = 111 - 8! x 4!