Question

Half life of a radioactive material is $5$ years, then the percentage of it remained after $25$ years will be

• A. 0.03125
• B. 0.0625
• C. 0.0125
• D. 0.25

Answer 1

Answer: (A)

0.03125

Answer 2

Here, $t = 25$ years, $T_{1/2}=5$ years Number of half lives, $n =\frac{t}{T_{1/2}}=\frac{25}{5}=5$ Percentage of radio active material remained after $25$ years is $=\left(\frac{1}{2}\right)^{n}\times100$ $=\left(\frac{1}{2}\right)^{5}\times 100 = 3.125\%$