Law of Radioactive decay;

N = N0e- λ t

where
N = number of active nuclei at time t

N0 = number of active nuclei at t = 0.

λ = decay constant


-dN/dt = λN

-dN/dt gives the number of decays per unit time and is called the activity (A) of the sample

A = λN

A = A0e- λ t