the aquarium has dimensions of 16in and 8.5in and 10.5,
which sounds like a rectangular prism,
a rectangular prism has a volume of length height width
so the volume of this aquarium is
16 8.5 10.5
is 3/4 full now, that is, 3/4 of 168.510.5
how much much water it needs to get filled?
well, is 3/4 full, to get 4/4, all you need is 1/4\(\bf \cfrac{1}{4}\ of \ (16\cdot 8.5\cdot 10.5)\implies \cfrac{1}{4}\cdot (16\cdot 8.5\cdot 10.5)
\\ \quad \\\\ \quad \\
\cfrac{16\cdot 8.5\cdot 10.5}{4}\)