Physics education research, electronic materials, and the musings of Christopher Moore, Ph.D.

2.71g sample of radioactive KCl is decaying at a constant rate of 400Bq into the isotope 40K. which constitutes 1.17% of normal K. calculate the half life of this nuclide.

Relevant equations
we have dn/dt = n * lamda and t-1/2 = 0.693/lamda.

how i put the value of N... which is not clear in question