Solution: Set $ \frac18D_0 = D_0 \cdot 2^-t/12 $. Divide both sides by $ D_0 $: $ \frac18 = 2^-t/12 $. Note $ \frac18 = 2^-3 $, so $ 2^-3 = 2^-t/12 $. Equate exponents: $ -3 = -\fract12 $. Solve for $ t $: $ t = 36 $. - go-checkin.com