The decay rate of a radioactive sample is called its activity. The half life of `U_92^238` against alpha decay is `1.5x 10^17s`. Calculate the activity of a sample of `U_92^238` having `25 x 10^21` atoms.

A radioactive substance decays to 1/32 of its initial value in 25 days. Calculate its half life.

A small amount of solution containing a radioactive nucleide A was administered into blood of a patient. The activity of the nucleide is 2 xx 10^(3) dps. Its half life is 15 hours. After 5 hours a sample of the blood drawn out from the patient. It's activity was 16 dpm per mL. Radioactive nuclide A^(x) decays by beta^(-) emission (42%) beta^(+) emission (58%) in the patient body Then half value of beta^(-) decay path is