Im going to give you the process so that you may find out with calculations what you need to do. It is simple the part that will be yours:
5 = 15e^(-.087t)
1/3 = e^(-.087t)
take ln both sides
ln(1/3) = -.087t
-1.1 = -.087t
t= ?
I think you can do the right calculations so that the answer of t is evident for you