In the third orbit of hydrogen atom the energy of an electron ' $E$ '. In the fifth orbit of helium $(Z=2)$ the energy of an electron will be

Ratio of longest wavelength corresponding to Lyman and Balmer series in hydrogen spectrum is

Half life of radio-active element is 1600 years. The fraction of sample remains undecayed after 6400 years will be

Frequency of the series limit of Balmer series of hydrogen atom in terms of Rydberg's constant (R) and velocity of light (c) is