Devise an exponential decay function that fits the given data, then answer the accompanying question. Be sure to identify the reference point $\backslash (($t$=0)\backslash )$ and units of time.

The homicide rate decreases at a rate of $5\backslash \%$ per year in a city that had $900$ homicides$/$yr in $2010.$ At this rate, when will the homicide rate reach $700$ homicides$/$yr$?$