An IQ Question

Now exam is over and school seems to be in a more “relax” mode, my 9 year old Genz Kid asked me an IQ question today.  

The Question:

Each bottle of soft drink costs $1.  With 3 empty bottles, you can exchange for 1 bottle of soft drinks.  If you have $10, how many bottles of soft drinks can you have?

The answer

…….  15