A mathematical model for the decay of radioactive substances is given by Q = Qoert where Q0 = amount of the substance at time t= 0, r = continuous compound rate of decay, t= time in years, and Q = amount of the substance at time t. An isotope has a half-life of 21 years. What is the continuous compound rate of decay? <: the continuous compound rate of decay is r="E." an integer or decimal rounded to four places as needed.>