Light of frequency 1.8 times the threshold frequency is incident on a photosensitive material. What will be the photoelectric current if the frequency is halved and intensity is doubled ? 

The ejection of phototelectrons depends on the full energy transfer by the impinging photon provided by the fact that the photon energy is sufficient enough to clear the work function at least. If the frequency is reduced then the transferred energy will not be enough to liberate the electron irrespective of the intensity of the incident beam.