Lifetime of a light bulb follows exponential distribution with $x$ means hours. If $n$ bulbs were switched on at same time and after $t$ time only $n-m$ were found to be functioning. Remaining $m$ bulbs had lifetime $x_{i}$ for $i=1$ to $m$. What will be the PDF of the bulbs that are still functioning?
My confusion is, since these $n-m$ bulbs are still functioning how can we find their PDF?