Remember the definition of density of "the amount of mass per unit volume".  Some of you may think that pound (lb) is for weight, not mass.  WRONG!!  We "mistakenly" use weight and mass inter-exchangeably.  Thus, you may say you weight 60 kg or 132 lb, which actually means that your mass is 60 kg or 132 lb.  Your "weight" (see the textbook for definition!) is actually the force "60 kg x g" or "132 lb x g" in which g is the acceleration of gravity = 9.81  m/sec² or 32.2 lb ft/sec², and the unit for weight is therefore kg m/sec² or lb ft/sec².