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A mathematical model for the Future Value of a savings account earning interest that is compounded continuously is given by the equation FV = Pert, where FV is the amount after t years, P is the principal amount invested at t = 0, and the principal is assumed to grow continuously at a rate, r. 

How many years will it take the principal to triple if the annual rate is 12%?  Please explain how you arrived at your answer. Thanks!