A15-g sample of radioactive iodine decays in such a way that the mass remaining after t days is given by m(t) = 15e^-0.087t, where m(t) is measured in grams. after how many days is there inly 5 g remaining?

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