Answer
V = 50.4 gal ( to three significant figures)
Work Step by Step
First we have to find the volume of aquarium than convert it to gallons.
V = length x width x height
V = 47.8 in x 12.5 in x 19.5 in
V = 11,651.25 $in^{3}$
We can express this in gallons using conversion factor: $\frac{1 gal}{231 (in^{3})}$
V = 11,651.25 $in^{3}$ x $\frac{1 gal}{231 (in^{3})}$
V = 50.438 gal
V = 50.4 gal ( to three significant figures)