Key idea If a quantity halves every t1/2t_{1/2}t1/2 years, then: Q(t)=Q0(12)t/t1/2Q(t)=Q_0\left(\frac{1}{2}\right)^{t/t_{1/2}}Q(t)=Q0(21)t/t1/2 Here t1/2=2t_{1/2}=2t1/2=2, so: Q(t)=Q0(12)t/2Q(t)=Q_0\left(\frac{1}{2}\right)^{t/2}Q(t)=Q0(21)t/2