# SOLUTION: The half-life of a substance is the time it takes for half of the substance to remain after natural decay. Radioactive water (tritium) has a half-life of 12.6 years. How long will

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 Question 202639: The half-life of a substance is the time it takes for half of the substance to remain after natural decay. Radioactive water (tritium) has a half-life of 12.6 years. How long will it take for 85% of a sample to decay?Answer by ankor@dixie-net.com(15652)   (Show Source): You can put this solution on YOUR website!The half-life of a substance is the time it takes for half of the substance to remain after natural decay. Radioactive water (tritium) has a half-life of 12.6 years. How long will it take for 85% of a sample to decay? : The half life formula: A = Ao where: A = the resulting amt after t (yrs in this case) Ao = initial amt t = time (yrs) h = half-life of substance (yrs) : Let initial amt: Ao = 1, then find A: 1.0 - .85 = .15 : 1*2^(-t/12.6) = .15 Find the log of both sides .301 = -.8239 = -.8239 Multiply both sides by 12.6 -.301t = -.8239 * 12.6 : -.301t = -10.381 t = t = 34.49 yrs : : Check solution on a calc: enter 2^(-34.49/12.6) = .1499 ~ .15