10 chairs are arranged in a row and are numbered 1 to 10. 4 men have to be seated in these chairs so that the ending chairs in the row can never be empty. In how many ways can this be done?
First select any two men from the four and arrange them in the ending seats in 4C2*2!
Then select two seats out of the 8 seats and arrange the two men in that. The number of ways that this can be done is 8C2*2!
So, the total number of ways in which this can be done is 8C2*2! *4C2*2! = 672