\(500=700\left(\frac{1}{2}\right) \frac{t}{T_{1 / 2}}\)
\(0.7 \approx\left(\frac{1}{2}\right) \frac{t}{T_{1 / 2}}\)
\(\left(\frac{1}{2}\right)^{1 / 2} \approx \frac{t}{T_{1 / 2}}\)
\(\frac{30}{\mathrm{T}_{1 / 2}} \approx \frac{1}{2} \Rightarrow \mathrm{T}_{1 / 2}=60\)