The mean lives of a radio-active substances are 1620 year and 405 years for α-emission and β-emission respectively. Find out the time during which three-fourth of a sample will decay if it is decaying both by α-emission and β-emission simultaneously.
Let at some instant of time t, number of atoms of he radioactive substance are N. It may decay either by α-emission or β-emission. So, we can write,
If the effective decay constant is λ, then
