You're lucky this is the last question I'm answering today :)
Part a
The vehicle travels 55 miles per hour, so for each hour "t" we will travel 55 miles.
[tex]d(t) = 55t[/tex]
Each 20 miles take 1 gallon, so for each gallon of fuel, we can travel 20 miles
[tex]d(g) = 20g[/tex]
Part b
In each hour we travel 55 miles, and each 20 miles require 1 gallon of fuel.
So,
[tex]55t = 20g \\ g = \frac{55}{20} t \\ g = 2.75t[/tex]
or you could set both d(t)=d(g) since they both are distance traveled.
Part c
[tex]g = 2.75(6) = 16.5 \: gallons[/tex]
[tex]d(6) = 55(6) = 330 \: miles \: \\ or \: \\ d(16.5) = 20(16.5) = 330 \: miles[/tex]
