Strontium-90 has a half-life of 28.8 years…

Step 1. The half-life in seconds. $T = 28.8 \times 3.156 \times 10^{7}\ \text{s} = 9.09 \times 10^{8}$ s.

Step 2. The decay constant. We want $\lambda$, so $\lambda = \dfrac{0.693}{T} = \dfrac{0.693}{9.09 \times 10^{8}\ \text{s}} = 7.63 \times 10^{-10}\ \text{s}^{-1}$.

Step 3. The mean life. $\tau = \dfrac{T}{0.693} = \dfrac{28.8\ \text{y}}{0.693} = 41.5$ y.

Step 4. After $100$ years. $\dfrac{N}{N_0} = 2^{-100/28.8} = 2^{-3.47} = 0.090$, so $9.0\ \%$ is left.