Answer
24.2 years
Work Step by Step
Follow the Half Life Model:
$$\frac{1}{2}=e^{kt}$$
As the half life time is 5.2,
$$\frac{1}{2}=e^{5.2k}$$
Solve for $k$:
$$\rightarrow k=-0.133$$
Follow the Exponential Decay:
$$P=P_0e^{kt}$$
As $P=2.4,P_0=60$
$$2.4=60e^{-0.133t}$$
$$\rightarrow t \approx 24.2$$ years
