Answer
$\Delta$$E_{th}$= 30 Joules
Work Step by Step
The total work that you put into the system is +100 Joules. We are told that +50 Joules of kinetic energy and +20 Joules of potential energy were produced. The only other contributing factor to Work is heat (thermal) energy. Total work is equal to all of the factors added together, so thermal energy must be equivalent to the kinetic and potential energies subtracted from 100. 100-70= 30 Joules
