Just as an example, let's say I'm a rental company that rents tools
for long periods of time. I want to understand how many tools are
being rented, how many I have left in inventory still available to
rent, and when I am going to exhaust my inventory. So for instance, I
have the following three primary fields:

1. Number of tools out for rent.
2. Number of tools in inventory still available for rent.
3. Sales growth month over month.

So using the sales growth, I should be able to roughly predict when I
am going to run out of inventory (assuming my sales growth is going to
continue to rise.)

I can't wrap my head around the logic to be honest. The variable I
want is the number of days left until I am going to run out of tools
for run.

Any Ideas?