Half life definition is given as;
'No one can say for sure when a particular nucleus will decay but one can predict how many in a given sample will decay over time. Radioactive elements have a half-life. The half life of any given element is the time that is required for one half of the sample to decay.'
For a radio active element we can write;
`A = A_0*(0.5)^(t/t_(1/2))`
A = substance remaining
`A_0` = Inital amount of susbstance
`t_(1/2)` = half life of susbtance
For our question;
`A_0 ` =`8.7E^4`
A = `3.6E^-4`
t = 2
Apply these values in the equation and take ln both sides.
By solving the equation we can get `t_(1/2)` = 0.837 days
Approximately it can be considered as 20 hours.