Eva needs to dilute some acid. How much pure water should be added to 2 gallons of $80\%$ acid to yield $20\%$ acid ?

Question

$\begin{array}{1 1} a)\; 5\;gallons \\ b) \; 6\;gallons \\ c)\; 8\;gallons \\ d)\; 10\;gallons \end{array}$