Suppose a 12C beam is produced at a tandem Van de Graaff accelerator. The negatively charged 12C ions become positively charged after being stripped by the stripping foil at the terminal. Then the analyzing magnet after the accelerator is used to select the 12C beam with charge state of 3+ for an experiment. What should the terminal voltage of the accelerate be set to if the energy of the 12C beam is needed to be 20Mev?