The number of brothers is the hundreds digit,
the tens digit is the number of sisters,
and the units digit is the number of living grandparents.

Let's use algebra to explain it.

1. Take the number of brothers that you have.	Let x = the number of brothers.
2. Multiply by 2.	Then we get 2x
3. Add 3.	We have 2x + 3
4. Multiply by 5.	Now we get 10x + 15
5. Add the number of sisters.	Let y = number of sisters   Now we have 10x + y + 15
6. Multiply by 10.	We obtain 100x + 10y + 150
7. Add the number of living grandparents.	Let z = number of living grandparents   We get 100x + 10y + z + 150
8. Subtract 150.	We get 100x + 10y + z   so the hundreds digit is the brothers   the tens digit is sisters   and the units digit is grandparents