If we know the total in a system is 30 joules and we know the poteial energy is 20jolueswhat is the KE?

The total energy in a system is the sum of its potential energy (PE) and kinetic energy (KE). In this case, with a total energy of 30 joules and potential energy of 20 joules, we can use the formula: Total Energy = Potential Energy + Kinetic Energy. So, KE = Total Energy - PE = 30 J - 20 J = 10 joules.