The radioactivity of a sample of a radioisotope was measured twice. The second measurement made 8.60 minutes after the first one produced the result which was 0.124 of the original value. What is the half-life (in minutes) of this radioisotope?

Note: Radioactivity is proportional to the amount or mass of the radioisotope in a sample. As the radioisotope decays the radioactivity decreases. The dimensionless number given is the fraction of the radioisotope that has not decayed.