The gravitational potential energy of an object is given by
[tex]U=mg \Delta h[/tex]
where m is the mass of the object, g the gravitational acceleration, and [tex]\Delta h[/tex] is the height of the object with respect to a reference point.
In our problem, the potential energy is U=375 J and the altitude of the object is [tex]\Delta h=15 m[/tex], so by re-arranging the equation we can find the mass of the ball:
[tex]m= \frac{U}{g \Delta h} = \frac{375 J}{(9.81 m/s^2)(15 m)}=2.5 kg [/tex]
