Question

How many ways are there to select 25 books from a collection of 27 books? i am just studying today

Answer 1

notice, this will just be a Permutation.

$\bf _nP_r=\cfrac{n!}{(n-r)!}\qquad \qquad \stackrel{\textit{25 books off a total of 27}}{_{27}P_{25}=\cfrac{27!}{(27-25)!}} \\\\\\ _{27}P_{25}=\cfrac{27!}{2!}\implies 5444434725209176080384000000$

you can always check your calculator also for a [ ₙPᵣ  ] button.