



 
Hi Christy, Suppose you have a mass of $C$ units of a radioactive material with decay constant $k$ where the time is measured in year. After $t$ years the mass $A$ of radioactive material remaining is given by \[A = C e^{k \; t}\] You are told that for Radium, $k = 0.000428$ and hence \[A = C e^{0.000428 \; t}\] You are asked to find the time $t$ when $A = 0.15 C.$ Substitute $A$ into the equation above and solve for $t.$ Penny  


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