A tumor is injected with 0.1 grams of lodine-125, which has a decay rate of 1.15% per day. To the nearest day, how long will it take for half of the lodine-125 to decay?
days

hi,  

1 - 0.015 = 0.985

so we have :  0.1 *0.985 ^n = 0.05

                              0.985^n =  0.05 /0.1

                              0.985^n = 0.5

                                        n =  ln (0.5) /ln (0.985)

                                         n ≈ 46

Let's check :   0.1*0.985^46 ≈0.0498