Want your Monthly Salary equal to the sum ?

Let us think about the combinations in ways of arranging the digits. There would be $9!$ ways to arrange any digit at any given place. Therefore there would be $9!$ of each $1,2,3...9$ (Zero doesn't contribute to the final digit sum.)

So the total value at each given place is $9! \times (1+2+3....+8+9 )$ , and these can be arranged in any of the 10 places (units to billions place).

So the sum of all the 10-digit numbers would be : $9! \times 45 \times (1+10+....+1000000000)$ $9! \times 45 \times (1111111111)$ $18143999998185600$ (Yes, I used a calculator for the above step, I don't know if there's an easier way to get to the direct digit sum rather than calculate it )

The digit sum of which is : $90$