Answer
Please see the work below.
Work Step by Step
We know that it will take Cesium 137 three times half life of 30.2y, or 90.6y to decay $\frac{1}{8}$ and its original mass is $1$ to $\frac{1}{2}$to $\frac{1}{4}$ to$\frac{1}{8}=3$ half lives.
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