SAT Math Multiple Choice Practice Question 31: Answer and Explanation
9. A certain radioactive element has a half-life of 20 years. Thus, a sample of 100 grams deposited in 1980 would have decayed to 50 grams by 2000 and to 25 grams by 2020. How much of this sample would remain in 2100?
A. 0 grams
B. 25/16 grams
C. 21/2 grams
D. 31/8 grams
E. 5 grams
Correct Answer: B
B. The easiest way to solve this problem is to make a table, dividing by 2 every 20 years:
The final answer, 1.5625, is the same as 25/16.