Dual Nature of Radiation

In a photoemission experiment, the maximum kinetic energies of photoelectrons from metals P,Q and R are EP, EQ and ER, respectively, and they are related by EP=2EQ =2ER*. In this experiment, the same source of monochromatic light is used for metals P and Q while a different source of monochromatic light is used for the metal R. The work functions for metals P,Q and R are 4.0eV,4.5eV and 5.5eV, respectively. The energy of the incident photon used for metal R, in eV, is________.