Shipping box is a cube, with units
Fish food is an unitary cube, that means have x,y,z measures the same
Shipping box is a parallelepip box. Rectangular
Volume of shipping box is found by formula
V = X•Y•Z
X = 3ft, Y=3 3/4 ft, Z=3 1/4 ft
then V = (3•3+ 3•3/4)•(3+1/4) = (11 +1/4)• (3+1/4) = 33 + 11/4 + 3/4 + 1/16
V = 33 + 2 + 3/4+ 3/4 +1/16 = 35 + 25/16 = 34+ 9/16
Now, find volume using packing cubes
How many fish food boxes fit
In X direction it fits 3/(1/4) = 12
In Y direction it fits (3+3/4)/(1/4)= 15
In Z direction it fits (3+1/4)/(1/4) = 10
Now then multiply 12x15x10= 1800 fish boxes fit in larger box
of the bathtub in one minute and faucet two fills up 
of the bathtub in one minute. If both are one then they will fill up 
So, 1/6 of the bathtub in one minute.