What is the half-life of a sample where the activity drops from 1,200 Bq down to 300 Bq in 10 days?
Half of 1,200 is 600, half of 600 is 300. So it takes two half-lives to drop from 1,200 Bq to 300 Bq, which is 10 days. So one half-life is five days.