An element whose half-life is 3 years has N atoms. How many atoms would have decayed after 9 years?

A. 1 N atoms 8
B. 1 N atoms 3
C. 2 N atoms 3
D. 5 N atoms 6
E. 7 N atoms 8

Correct Answer:

Option E – 7 N atoms 8

Explanation

N/3 = 3years

N/4 = 6years

N/8 = 9years

N – N/8 = 7N/8