15) 4 minutes
Initial count rate = 140 – 20 (background radiation) = 120
Final count rate = 35 – 20 = 15
We need to figure out the number of half lives.
120 → 60 = 1 half life
60 → 30 = 2 half lives
30 → 15 = 3 half lives
There were three half lives over the period of 12 minutes
Time per half life = 12 ÷ 3 = 4 minutes