Differential Equations: JEE Main 2012 Mathematics question

The population $p$ $(t)$ at time $t$ of a certain mouse species satisfies the differential equation ${{dp\left( t \right)} \over {dt}} = 0.5\,p\left( t \right) - 450.\,\,$ If $p(0)=850,$ then the time at which the population becomes zero is :