Potential energy is the energy possessed by a body or an object while at rest while kinetic energy is the energy while the body is at motion.
Potential energy is given by Mgh where m is the mass of the object, g is the gravitational acceleration (a constant) and h is the height.
Therefore, potential energy varies directly proportional to the mass of the object and the height.
Thus in this case, an increase in the mass of the box by 15 kg and raising the height of the box by 20 feet, will result to an increase in potential energy of the box.