Math  Logorithms
posted by Labz .
The intensity, I, of light, in lumens, passing through the glass of a pair of sunglasses if given by the equation I(x) = I^o(0.8)^x, where x is the thickness of the glass, in millimeters, and I^o is the intensity of light entering the glasses. How thick should the glass be so that it will block 25% of the light entering the sunglasses?

We want
I^o(.25) = I^o(.8)^x
.25 = .8^x
ln.25 = x*ln.8
x = ln.25/ln.8 = 6.21