A radioactive element has half life period 800 years. After 6400 years what amount will remain?

Starting from 10g of a radioactive element, 0.25 g was left after 5 years. Calculate The amount left after one year.

Strontium-90 is a radioactive isotope having a half-life of 28 years.Calculate the decay constant lambda(in s^(-1)) of Strontium-90,

4 g of radioactive material of half-life 10 years is kept in a store for 15 years.How much material is disintergrated?

After five half life periods for a first order reaction what fraction of reactant remains ?