1. Take the number of brothers that you have.

2. Multiply by 2.

3. Add 3.

4. Multiply by 5.

5. Add the number of sisters.

6. Multiply by 10.

7. Add the number of living grandparents.

8. Subtract 150.

9. What number do you have? ..

..

..

..

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.



Why does this work?

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