What percent of light will pass thru 25 panes

if a single pane of glass obliterates 10% of the light passing thru it, then the percent P of light that passes thru X successive panes is given approximately by the equation P=100E^-0.1x.

a. what percent of light will pass thru 25 panes?

b. How many panes are necessary to block at least 75% of the light?

I don't know where you got that formula.

answer is 0.9^25 = 0.0718 = 7.2%

block 0.75 means pass 0.25
0.9^x = 0.25
x log 0.9 = log 0.25
x = -0.602 / -0.0458 = 13.2 panes, so you need 14 panes.

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a. 100e^(-0.1*25)

b. Solve 100e^(-0.1x) = 75 for x.