Nuclei

Total time given= 80 min
Number of half-lives of A,
Number of half-lives of A,
Number of nuclei remains undecayed

where is initial number of nuclei.

or
or

decay can involve the emission of either electrons or positrons. The electrons or positrons emitted in a decay do not exist inside the nucleus. They are only created at the time of emission, just as photons are created when an atom makes a transition from higher to a lower energy state. In negative decay a neutron in the nucleus is transformed into a proton, an electron and an antineutrino. Hence, in radioactive decay process, the negatively charged emitted particles are the electrons produced as a result of the decay of neutrons present inside the nucleus.

(anti-neutrino)

From Rutherford-Soddy law, the number of atoms left after n half-lives is given by

where, is original number of atoms.
The number of half-life
Relation between effective disintegration constant
and half-life (T) is

Effective half-life