perm (1 Viewer)

[1zephyr]

in how many ways can seven boys be arranged in a row so that a and b never sit next to each other

 

[photastic]

Not together = all - together = 7! - 6!2!

Explanation:
To arrange 7 people: _ _ _ _ _ _ _ there is 7 x 6 x 5 x 4 x 3 x 2 x 1 ways
To arrange 2 people together" + + _ _ _ _ _ arranging actually 6 objects, the 5 people and the two people but there is 2! ways to arrange the two people, hence 6! x 2!