At the beginning of an experiment, a scientist has 296 grams of radioactive goo. After 120 minutes, her sample has decayed to 9.25 grams.
the amount left after time t minutes is
A = 296 e^(-kt)
You know that 296 e^(-120k) = 9.25
that will help you answer whatever question you are reluctant to ask.
Note that 9.25/296 = 1/32 = 1/2^5