Answer
At the end of 90.0 years, the fraction of the original sample that remains is $~~\frac{1}{8}$
Work Step by Step
At the end of 30.0 years, half of the sample has decayed and half of the original sample remains.
At the end of 60.0 years, the fraction of the original sample that remains is $(\frac{1}{2})^2 = \frac{1}{4}$
At the end of 90.0 years, the fraction of the original sample that remains is $(\frac{1}{2})^3 = \frac{1}{8}$
