What is the unit of Radioactivity?

The activity of a sample of a radioactive isotope that decays into a stable nucleus fell from 137Bq to 125Bq during an hour. Calculate: the half-life of the isotope; the number of nuclei of the radioactive isotope in the sample at the beginning and end of the hour; the mean lifetime of a nucleus of the radioactive isotope; how long must elapse until just one radioactive nucleus is left in the sample.