Example

Try It 10.90: Evaluating an Exponential Decay Model Using Half-Life

Further practice solving an exponential decay problem using half-life. Suppose the half-life of radioactive iodine is 6060 days, and we want to determine how much of a 5050-mg sample will remain in 4040 days. Using the exponential decay formula A=A0ektA = A_0 e^{kt}, find the decay rate kk by noting that half the sample (2525 mg) remains after 6060 days: 25=50ek6025 = 50 e^{k \cdot 60}. Dividing by 5050 gives 0.5=e60k0.5 = e^{60k}. Taking the natural logarithm of both sides gives ln0.5=60k\ln 0.5 = 60k, resulting in k=ln0.560k = \frac{\ln 0.5}{60}. Substitute this rate back into the formula to find the amount after 4040 days: A=50eln0.56040A = 50 e^{\frac{\ln 0.5}{60} \cdot 40}. Evaluating this expression yields approximately 31.531.5 mg. Therefore, about 31.531.5 mg of the sample will be left.

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