According to the  law of radioactivity, the rate of decay is proportional to the original number

note replace H with λ  and v in the above equation with a proportionality symbol α

Taking the  recIprocal from both side we have

 but let D =

this is the equation for the determination of ages of rocks, where t is the age of the rock, D is the daughter nuclei and N is the parent nuclei , note H is the decay constant.