It appears that the line crosses through the point $(35 \mbox{ min}, 200 \mbox{ counts/min})$ . It is also clear that if you continued the plotted line to the right of the graph, it would intersect the $100 \mbox{ counts/min}$ horizontal line somewhere between $40 \mbox{ min}$ and $45 \mbox{ min}$ . Therefore, the half-life must be between $5 \mbox{ min}$ and $10 \mbox{ min}$ . Therefore, (B) is the only answer that could possibly be correct.

Solution to 1992 Problem 26