A properly exposed photograph is taken at f/16 and 1/120s. What lens opening is required if the shutter speed is 1/60s

f / 8

Explanation:

The f-number is the ratio of the lens's focal length to its diameter and is a measure of the intensity of the light reaching the CCD. The smaller the greater the intensity of the light

           I = D / f

           f-number = f / D

           I = 1 / f-number

the energy deposited in the CCD is proportional to the intensity of the light and the exposure time (Δt)

          E = I Δt

          E = 1 /f-number   Δt

in the exercise they indicate that the system is well exposed (the image is sharp and clear) for fo-number = f / 16

        Δt = exposure time = 1/120 s

        E =    

indicate that the exposure time has been changed to Δt = 1/60 s, which should be the f₁-number

        E =

the energy deposited must be the same per location we can equal the expressions

         f₁-number =

         f₁-number = ½  16

         f₁-number  = 8

so the system should be set to f / 8

i don't know that i agree with that but what's the question ?