so it went from 1/3 full to 3/4 full when 10 gallons were put in, so.... what fraction is 10 gallons anyway? let's check the difference.
if say, when full is "x" gallons,
[tex]\bf \cfrac{3}{4}x-\cfrac{1}{3}x=10\implies \cfrac{9x-4x}{12}=10\implies \cfrac{5x}{12}=10\implies 5x=120
\\\\\\
x=\cfrac{120}{5}\implies x=24[/tex]
