Question

A sample of radioactive material initially contains 1.2 x 1020 atoms of the isotope. The decay constant for the isotope is 3.6 x 10-3s-1. Calculate the number of atoms of the isotope remaining after 1000s.

Answer 1

This is a straightforward application of the equation for radioactive decay\[N(t)=N(0)\exp(-\lambda t)\]
where N(t) is the number of undecayed atoms at time t and lambda is the decay constant.