Question

If 80% of a radioactive element undergoing decay is left over after a certain period of time t from the start, how many such periods should elapse from the start for the 50% of the element to be left over ?

• A. 4
• B. 6
• C. 5
• D. 3

Answer 1

The correct option is D

3

$t_{20}=\frac{2.303}{\lambda }log\frac{100}{100-20}=\frac{2.303}{\lambda }log\frac{10}{8}=\frac{2.303}{\lambda }[1-3log2]t_{50}==\frac{2.303}{\lambda }log\frac{100}{100-50}==\frac{2.303}{\lambda }log2\frac{t_{50}}{t_{20}}=\frac{log2}{1-3log2}=or{t}_{50}=3t_{20}=3t$