Question

Find the greatest number which divides the greatest $3$-digit number and the greatest $4$-digit number.

Answer 1

Greatest $3$-digit number = $999$

Greatest $4$-digit number = $9999$

Divisibility rule for $9$ = if the sum of digits of a number is divisible by $9$

Hence, $9+9+9=27$ which is divisible by $9$

And $9+9+9+9=36$ which is also divisible by $9$

Hence we can say that,

greatest number which divides the greatest $3$-digit number and the greatest $4$-digit number is $9$.