The formula for calculating the amount remaining after a number of half years , n is :
[tex]\begin{gathered} A=\frac{A_{\circ}}{2^n^{}} \\ \text{where A}_{\circ}\text{ =initial }amount \\ n=\frac{t}{t_{\frac{1}{2}}} \end{gathered}[/tex]The lab store mass of Cesium-137 is 30mg in 1975
then the mass of Cesium-137 in 2065,
Time period =2065-1975
time period t=90 years,
substitute the value and solve for A
[tex]\begin{gathered} A=\frac{30}{2^{\frac{90}{45}}} \\ A=\frac{30}{2^2} \\ A=\frac{30}{4} \\ A=7.5\text{ mg} \end{gathered}[/tex]In 2065, the mass of Cesium -137 will be 7.5 mg
Answer : 7.5mg
