Find the number of permutations of n different things taken r at a time such that two specific things occur together.

Please explain in detail.

A bundle of 2 specific things can be put in r places in (r – 1) ways and 2 things in the bundle can be arranged themselves into 2 ways. Now (n – 2) things will be arranged in (r – 2) places in ways.
 

Thus, using the fundamental principle of counting, the required number of
permutations will be