Answer:
30 grams
Step-by-step explanation:
If the half life is 21 hours, that means after 21 hours, the initial amount will decrease to half.
So, since we started with 120 grams, it will be at 60 grams after 21 hours.
After another 21 hours have passed (after a total of 42 hours), the amount of 60 grams will split in half again to 30 grams.
So, after 42 hours, there will be 30 grams left of the radioactive iodine
