The graph shows the number of nuclei N remaining as a function of time. The point on the graph that corresponds to the number of nuclei remaining after two half-lives have elapsed is
A) 1
B) 2
C) 3
D) 4
E) 5

Question

The graph shows the number of nuclei N remaining as a function of time. The point on the graph that corresponds to the number of nuclei remaining after two half-lives have elapsed is
A) 1 
B) 2 
C) 3 
D) 4 
E) 5

Quizplus · Accepted Answer

After one half-life, the number of nuclei remaining is halved (i.e. N/2). After two half-lives, the number of nuclei remaining is halved again (i.e. N/2 * 1/2 = N/4).
Looking at the graph, it can be seen that N/4 corresponds to the point where the curve intersects the x-axis for the third time, which is at point C. Therefore, the correct answer is choice C.

The Graph Shows the Number of Nuclei N Remaining as a Function