Question

There are $5$ books on Mathematics, and$4$on physics. In how many ways these be placed on shelves if the books on the same subject area to be together?

Answer 1

The number of ways these are placed on shelves if the books of same subject to be together:

Number of ways to arrange mathematics books together $=5!$
Number of ways to arrange physics books together$=4!$

The total number of ways is calculated as follow:
$\begin{array}{rcl}\mathrm{Toatal}\mathrm{number}\mathrm{of}\mathrm{ways}& =& 5!\times 4!\times 2\\ & =& 5760\end{array}$

Hence, the number of ways these are placed on shelves if the books of same subject to be together is $5760$.