For a first order reaction the rate R = -k N so the ratio of rates at two different times is given by the R(t1)/R(t2) = N(t1)/N(t2).
A chemist determines that a sample of petrified wood is 1.00X 103 years old. For nuclear decay the rate is often is expressed in units of counts per unit of time. Determine the rate of decay, in counts per minute per gram, of carbon-14 in the sample? The decay rate ofcarbon-14 in wood today is 13.6 counts per minute per gram, and the half life of carbon-14 is 5730 years.