A certain radioactive element disintegrates for an interval of time equal to its mean life. What fraction of the element has disintegrated?

Starting from 10g of a radioactive element, 0.25 g was left after 5 years. Calculate The time required for half of the element to decay.

The half - life period of radioactive element A is the same as the mean life time of another radioactive element B. Initially both of them have same number of atoms. The radioactive element B decays faster than A. Explain, why.

Half life of a certain radioactive material is 130 days. After what lapse of time, the undecayed fraction of the material will be 25% ?

Half life of a certain radioactive material is 140 days. After what lapse of time, the undecayed fraction of the material will be 25% ?

A radioactive uclide has a decay costant equal to lambda .Give the formula for the mean life of this nuclide.