Question

calculate speed of light in medium whose frequency and wavelength aree 4000A° and 12× 10^2Hz respectively

Answer 1

we know, speed of light = frequency * wavelength
therefore c=4000*10^(-10) * 1200
                 =4.8*10^(-5)  (answer !)

Answer 2

We know that speed of light is multiple of wavelength and frequency
c=wavelength *frequency
here
wavelength=4000A=4000*10^-10=4*10^-7m
frequency is 12*10^2Hz=1/12*10^2sec
multiple both
4*10^-7/12*10^2=3*10^-10ms^-1
hope it help