Find the number of permutations of n distinct things taken r together, in which 3 particular things must occur together.

Explanation

Given `r` places, we first fill up 3 places by 3 particular things. This can be done in ` r_(P_3)` ways.

Now, we have to fill remaining `r-3` places with remaining `n-3` things.
This can be done in ` (n-3)_(P_(r-3))` ways.
Thus, the required number of permutations will be ` r_(P_3) times (n-3)_(P_(r-3))` ways.