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

Explanation

Total number of things = n
We have to arrange r things out of n in which three things must occur together.
Therefore, combination of n things taken r at a time in which3 things always occurs
`= ""^(n-3)C_(r-3)`
If three things taken together, then it is considered as 1 group.
Arrangement of these three things `=3"!"`
Now, we have to arrange `= r - 3 + 1 = (r - 2)` objects
`therefore" "` Arranged of (r-2) objects `= r - 2"!"`
`therefore" "` Total number of arrangements `= ""^(n - 3)C_(r - 3) xx r - 2"!" xx 3"!"`