Question

How many number of $5-$digit telephone numbers be formed having atleast one of their digits repeated.

Answer 1

No:of Digits in telephone no = 5

No:of digits with repletion $=(10{)}^5$

No:of digits without repetition $10\times 9\times 8\times 7\times 6$

$\Rightarrow 30240$

No : of no with at least are digit replicated

$\Rightarrow 100,000-30,240$

$\Rightarrow 69,760$

$Re{q}^{d}No:69760$