Answer:
11.29 grams of strontium-90 were present in the original rock sample in 1933.
Step-by-step explanation:
Strontium has a half-life of 28.8 years. Therefore,
1989 - 1933 = 56 years.
56 / 28.8 = 1.94 half-lives
Thus, the quantity of the radioisotope remaining will double for each half-life elapsed. moving backwards in time.
Therefore, moving backwards in time by 1.94 half-lives, the quantity remaining will double by 1.94 times.
Thus, the amount remaining in 1933 is
3.0 × (1.94)² = 11.29 grams
