As this is an exam question I do not feel comfortable working out the answer for you without some working. My answer is 72 gallons for reference
Consider the 600 gallons as pure HCl and impure solvent. You can consider the percentage concentration as vol/vol meaning that x% means x gallons HCl per gallon solvent.
If you get the amount of HCl in there as though it were pure HCl, you'll get a value in L.
Do the same for the 27% and the difference should be how much you need to put in