From the statement, the wavelength should be 1.6 cm * 1/4 = 0.4 cm. Then, we use the wave equation formula. The frequency is equal to the speed of light divided by the wavelength. First, let's use the SI units.
0.4 cm = 0.004 m
The speed of light is equal to 3 x 10^8 m/s
Frequency = 3 x 10^8 m/s ÷ 0.004 m
Frequency = 7.49 x 10^10 s^-1