Radioactive disintegration follows first order reaction kinetics, such that, `t=1/lambdaln(a/(a-x))`
where the terms have their usual significance.
Half-life is the time required to disintegrate into half the original mass of a radioelement.
Put this value of disintegration constant in the second condition:
`=60 ` days
Therefore, it would require 60 days to disintegrate 3/4 th of the original mass of the radioelement.