### Theory:

The greatest one-digit number is $$9$$. Adding $$1$$ to the greatest one-digit number will result in  $$9+1 =10$$. That is, it results in the smallest two-digit number.

The greatest two-digit number is $$99$$. Adding $$1$$ to the greatest two-digit number will result in $$99+1 =100$$. That is, it results in the smallest three-digit number.

Proceeding in this way, we can have the following table.

 Greatest number Adding 1 Smallest number $$9$$ $$+1$$ $$=10$$ $$99$$ $$+1$$ $$=100$$ $$999$$ $$+1$$ $$=1000$$ $$9999$$ $$+1$$ $$=10000$$ $$99999$$ $$+1$$ $$=100000$$ $$9999999$$ $$+1$$ $$=1000000$$ $$9999999$$ $$+1$$ $$=10000000$$

Thus, the resultant pattern becomes,
Greatest single$$(1)$$ digit number $$+ 1 =$$ smallest $$2$$-digit number
Greatest $$2$$-digit number $$+ 1 =$$ smallest $$3$$-digit number
Greatest $$3$$-digit number $$+ 1 =$$ smallest $$4$$-digit number
and so on.
Important!
Recall the following:
$$1$$ tens $$= 10$$ ones
$$10$$ tens $$= 1$$ hundreds $$= 100$$ ones
$$1$$ thousand $$= 10$$ hundreds $$= 100$$ tens
$$1$$ lakh $$= 100$$ thousands $$= 1000$$ hundreds
$$1$$ crore $$= 100$$ lakhs $$= 10,000$$ thousands