Math

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The aperture setting, or f-stop, of a digital camera controls the amount of light exposure on the sensor. Each higher number of the f-stop doubles the amount of light exposure. The formula n=log_2(1/p) represents the change in the number, n, of the f-stop needed, where p is the amount of light exposed on the sensor.

a) a photographer wishes to change the f-stop to accomodate a cloudy day in which only 1/4 of the sunlight is available. How many f-stops does the setting need to be moved?

My answer:
2 (which is correct)

b) if the photographer decreases the f-stop by four settings, what fraction of the light is allowed to fall on the sensor?

I need help on part b. My attempt is this:
-4p=log_2(1/p)
then I solved for p, and got -1/2 but it is wrong.

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