1) after 1 layer , we have 1-0.2 =0.8 passing thru

so 0.8 get to travel to the 2nd layer, where another 0.2 will be cut out

2) so it works in the same way as compound interest , only difference is that it "depreciates"

and a similar example would be

if the interest rate is 5% p a, how many years would a principal of P double if the interest if compounded annually ?

so P*(1.05) is the money after 1 year, P*(1.05)*(1.05) is the money after another year

after n years , we have P*1.05^n which should equal 2*P

so 1.05^n=2

take log of both sides

ln (1.05^n)= ln2

n *ln 1.05= ln2

n = ln2 / ln 1.05 = 14.2066...=15 for it to double as it only calculates interest annually

The question you're asking is very similar, but it has inequality in it

so be carefully with the inequality signs

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