Radioactive decay is expressed by the following formula: [tex] N_{t} = N_{0} e^{-lambda * t} [/tex] N₀ is the initial number of undecayed atoms. Nt is the number of undecayed atoms remaining after time t λ is the decay constant. The relationship between λ and the half life time t1/2 is: λ = [tex] \frac{0.693}{ t_{1/2} } [/tex] = 0.693 / 5.27 = 0.1315 [tex] y^{-1} [/tex] Taking natural logs for both sides of the decay expression: ln Nt = ln N₀ - λ t ln Nt = ln 199 - (0.1315 x 20) = 5.293 - 2.63 = 2.66 From which: Nt = 14.29 g
