Final answer:
After two half-lives (40 minutes), half of the remaining radioactive bismuth will have decayed twice. Starting with 72 grams, the amount left after 40 minutes is 18 grams.
Step-by-step explanation:
To determine how much of a radioactive bismuth is left after 40 minutes, knowing that you start with 72 grams and the half-life is 20 minutes, you can use the concept of half-life, which indicates how much time it takes for half of the radioactive material to decay. After one half-life (20 minutes), you would be left with half of the original amount. After two half-lives (40 minutes), you would have half of that amount again.
Let's calculate it step by step:
- After 20 minutes (1 half-life), 36 grams will remain (72g / 2).
- After another 20 minutes (now a total of 40 minutes, which is 2 half-lives), half of the remaining 36g will have decayed, leaving us with 18 grams (36g / 2).
Thus, the answer is b) 18 grams.