Find the number of 10-digit numbers, where the sum of the digits is divsible by $9$.
First 10-digit number divisible by 9 =1,000,000,008
Last 10-digit number divisible by 9 =9,999,999,999
Subtract Last from First, divide by 9 and add 1
[9,999,999,999 - 1,000,000,008] / 9 + 1 =1,000,000,000 ten-digit numbers divisible by 9.
