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t in (-oo:+oo)
(1/5)*e^-4*t = 0.1 // - 0.1
(1/5)*e^-4*t-0.1 = 0
1/5*e^-4*t-0.1 = 0 // + 0.1
1/5*e^-4*t = 0.1 // : 1/5*e^-4
t = 0.1/(1/5*e^-4)
t = 0.5*e^4
t = 0.5*e^4
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