But a problem that can be solved by comparing feet in inches by using a model includes solution explain why you were changing from larger unit into a smaller unit

Wow, that is hard to understand exactly what is being said.

You change larger units into smaller units so that the unit sizes match and you are comparing "apples to apples".  This makes it easier to see which one is larger or if they are equal.  

Does that help?

2.) is the most sense