Strontium$-90$ decays according to the exponential function $y=y_0{e}^{-0\mathrm{.}0241t},$ where $t$ is time in years.

Solve $0\mathrm{.}9y_0=y_0{e}^{-0\mathrm{.}0241t}.$

Evaluate $y=200e^{-0\mathrm{.}0241(30)}$

Solve $\frac{1}{2}{y}_0=y_0{e}^{-0\mathrm{.}0241t}.$

Evaluate $y=200e^{-0\mathrm{.}0241(15)}$