Question

a radioactive substance has a mean life of 20 years

Answer 1

Since the Question is not specific about what is to be answered thus answering about the radioactivity and mean life.

Many substances found in nature are radioactive i.e. they decay over time and convert to another element. This decay takes place by the change of molecular composition of the compound and loss of energy through radiation. radioactive decay is a random process and therefore any atom of the composition may decay any time. Decays can be alpha decay, beta decay, gamma decay etc.

The half life of a radioactive substance is known as the amount of time required for the a particular amount of radioactive substance to decay to half of its initial amount i.e. half of the atoms decay in this time. Similarly, the mean life of a radioactive substance is known as the time required for the whole amount of radioactive substance to decay. Mean life of a radioactive substance is 1.443 times of the half life of the substance.