Given he has filled his truck with 12 gallons, the unused gas is \(\frac{12}{4}*3 = 9\), where 4 is the sum of the parts in the ratio and 3 the part unused.

Then, he can drive for \(9*20 = 180\) miles more.

Answer D

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
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