Respuesta :

The answer is:  30 ft³ ;  or, write as:  30 cubic feet .
________________________________________________________
Explanation: 
__________________________________________________
Given:
__________________________________________________
Density = 0.4 fish(es) per cubic foot (or, feet);
__________________________________________________
or, "Density = 0.4 fish / ft³ ;
__________________________________________________
And, given: "12 fish in the tank." ;
__________________________________________________
And Density = (value; in this case, "number of fish" / unit volume);

We are given:  Density, "D" = 0.4 fish / 1 ft³
______________________________________________
In our case, our volume is in "cubic ft." ; or, "ft³ ;
_______________________________________________
We want to find the actual volume of the tank (in "cubic feet"; or, " ft³ ");
_________________________________________________
 →  (1 ft³ / 0.4 fish) * 12 fish  = _____?  ft³ ?

 →  On the "left-hand side" of the equation; the units of "fish" cancel out to "1"; and we are left with units of "ft³ " ; which is the units for which we wish to solve.
_______________
  → So, so have  (1 ft³ * 12) / (0.4) = 12 ft³ / 0.4 = 30 ft³ .
___________________________________________________
  → The answer is:  30 ft³ ;  or, write as:  30 cubic feet .
_______________________________________________________