Half Life Questions

The activity of a radioactive sample is measured as $N_0\text{ counts per minute}$ at $t=0$ and $N_0/e\text{ counts per minute}$ at $t=5\text{ minutes}$. The time (in minutes) at which the activity reduces to half its value is:

A mixture consists of two radioactive materials $A_1$ and $A_2$ with half-lives of $20\text{s}$ and $10\text{s}$ respectively. Initially the mixture has $40\text{g}$ of $A_1$ and $160\text{g}$ of $A_2$. The amount of the two in the mixture will become equal after: