<u>Answer:</u> 25
<u>Step-by-step explanation:</u>
We know that if y (dependent variable) varies directly with x(Independent variable) then the equation for direct variation is given by :-
, where k is the proportionality constant.
Since, it is given that the volume ( dependent variable ) of a box(V) varies directly with its length(l) (independent variable) , then the equation will be
, where k is the proportionality constant.
If one of the boxes has a volume of 325 cubic inches and a length of 13 inches, then

Hence, the constant of proportionality for the group of boxes = 25