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Priyanshu kumar Best Answer
Assuming an exponential law of the light absorption like L=L0*exp(-a*x) one gets 0.9 = exp(-a*l) from which one obtains fraction of light passing the thickness 5l as exp(-a*5l) = (exp(-a*l))^5 = 0.9^5 = 0.59. So only about 60% of light will pass this thickness of 5l