Let us consider Bob uses x gallons

Therefore we can set the ratio as -

$\frac{(12-x)}{x} =\frac{3}{1}$ (since 12 gallons is full tank and (12-x) is the unused gas)

or $x= 3$.

Now we have to consider in the ratio of $\frac{3}{1}$

i.e $\frac{3x}{x} = \frac{9}{3}$ (putting the value of x = 3, because we have assumed he uses x gallons and so in every 3parts used gas we have 9 parts unsed gas)

Therefore Bob can travel with 9 gallons remaining fuel = 9*20 = 180 miles