Excel 2007 : Calculate many months of inventory

hi,
I am trying to write a formula or a macro which can be used to calculate how many months of demand can be covered based on current inventory.
For example,
Current month is 500
and the demand for the next 6 months is 100 each, then the formula should return 5 or if the inventory was 550 then for the same demand the formula should return 5.5.
Does excel provide any standard function for this calculation? can someone provide a simple formula or a macro which i can reuse to get the desired result?

thanks,