Find the number of permutations of n distinct things taken r together,

Given r places, we first fill up 3 places by 3 particular things. This can be done in rP3 ways.

Now, we have to fill remaining r − 3 places with remaining n − 3 things.

This can be done in n − 3Pr − 3 ways.

Thus, the required number of permutations will be rP3 × n − 3Pr − 3 ways.