By adding 1 to the greatest ____ digit number, we get ten lakh.
6
We need a number such that when we add 1, we get ten lakh.
The greatest n-digit number is made by writing all 9s.
For example: greatest 2-digit number = 99, greatest 3-digit number = 999.
If the blank is 6, then the greatest 6-digit number is 999,999.
Add 1 to it:
\(999{,}999 + 1 = 1{,}000{,}000\)
In the Indian system, \(1{,}000{,}000\) is ten lakh.
(Also called one million.)
So, by adding 1 to the greatest 6-digit number, we get ten lakh.
Therefore, the blank is 6.