Answer:
When Jeff travels more than 20 miles
Step-by-step explanation:
Let 'x' be the number of miles traveled by Jeff.
In order for the national car company to be less expensive, then the following condition must be met:
[tex]12+0.30x < 10+0.40x\\12-10 < 0.4x-0.3 x\\2 <0.1x\\20<x[/tex]
This means that when Jeff travels more than 20 miles, it is cheaper to rent from the national car company.