Question

How many numbers can be formed taking $4$ digits from digits $1,2,3,4,5,6$ whereas digits $4$ and $5$ are compulsory?

Answer 1

$[1,2,3,4,5,6]$

Where $,4$ and $5$ are compulsory then, we have,

$1,2,3,6$

$\Rightarrow$ it reputation of digit is no allowed then $2$ place can be tilled by

${\Rightarrow }^4{C}_2\Rightarrow \frac{4\times 3}{2}\Rightarrow 6$ ways and, each of the digits can $4!$ permutation

So, $6\times 4!\Rightarrow 6\times 24$

$\Rightarrow 144$ ways