Let the constant be x.
No. of deer,bears and foxes= 3x,7x,5x
Therefore,
(7x-3x)=7*3*y(where y is any constant)
or, 4x=21y
or, x is divisible by 21, since 4 is not divisible by 21
Therefore, min. value of x=21.
or, Min. no of animals= (7x+3x+5x)=15*21=315(Ans.)