A lorry of mass 10000 kg takes 5000 kg of sand to the top of a hill 50 m high,unloads the sand and then returns to the bottom of the hill.
The gravitational field strength is 10 N / kg. What is the overall gain in potential energy?

The change in gravitational potential energy in a constant gravity field is given by:

\Delta PE = mg ( \Delta h )

So - how much mass was moved, and how high, from beginning to end of this problem statement? Do the math and post back yuor answer so we can check it for you.