A radioactive sample had an initial activity of 28 disintegrations per minute. 30 minutes later, its activity became 14 disintegrations per minute. How many atoms of the radioactive nuclide were present intially ?

Explanation

`t_(1//2) = 30` min, `A = lambda N , t_(1//2) = (0.693)/(lambda) therefore lambda = (0.693)/(30 "min")`
Number of atoms present intiallly, `N = (A)/(lambda) = (28 "dis"//"min")/(0.693//30 "min") = 1200`