Question

How many telephone numbers are there which contain the combination $12$ if the number consists of $5$ digits?

Answer 1

The combination $12$ can be in four places

For each case, the remaining three places can be filled by $10$ digits

$\therefore$ $10\times 10\times 10=1000$ numbers can be formed

And there are $4$ cases for combination

So, total $4000$ numbers can be formed .