The expanded form of 15,409 would be 15,000 + 400 + 9.
Your 15 would alone be 15,000. The 409 is turned into zeros.
For 409, take the 9 off of the end and keep it by itself, and then add a zero in replace for the 9, which makes 400.
The 9 should come in at the last part as it is the last number.
Add them all together and you should get 15,000 + 400 + 9.