Answer:
32
Explanation:
Let $x$ represent the number of gallons the tank holds when it is full. We know that the difference between $\frac78$ full and $\frac12$ full is 12 gallons, so we set up an equation and solve for $x$.
12=(7/8)x-12x
12=(3/8)x
12*(8/3)=x
32=x
The tank holds $\boxed{32}$ gallons when it is full.