Find Glass Thickness to Block 25% of Light in Sunglasses

[fxacx]

The intensity, I of light in lumens, passing through the glass of a pair of sunglasses is given by the
equation I(x) = I0 (0.8)^x , where x is the thickness of the glass in millimetres and I0 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?

 

[skeeter]

H

The intensity, I of light in lumens, passing through the glass of a pair of sunglasses is given by the
equation I(x) = I0 (0.8)^x , where x is the thickness of the glass in millimetres and I0 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?

$0.75 I_0 = I_0 (0.8)^x$

solve for x ...

 

[HallsofIvy]

So the question is, again, do you know how to solve $a^x= y$? You titled this "Log/exponential word problem". Do you know what "logarithms" and "exponentials" are and how they are related?