5 days because there are 24 hours in a day so every six hours you can take 4 a day
ANSWER

EXPLANATION
The given expression is

We regroup the numerical parts to get;

Now let us simplify the negative numbers.

We now subtract 19 from 42 to get;

We can't simplify further
Hence the simplest form is:

$16.80 / 4 gallons = $4.20 per gallon
The answer is C
Hope it helped