Question

The half-life period of radioactive element if 75% of it disintegrates in 40 min is :

• A. 20 min
• B. 10 min
• C. 40 min.
• D. none of these

Answer 1

Answer: (A)

20 min

$1^{st}$ half- Life $\to$ 50% decay

$2^{nd}$ half- Life $\to$ 75% decay

$3^{rd}$ half- Life $\to$ 87.5% decay

$4^{th}$ half- Life $\to$ 93.75% decay

$5^{th}$ half- Life $\to$ 96.87% decay

$6^{th}$ half- Life $\to$ 98.44% decay

$7^{th}$ half- Life $\to$ 99.22% decay

Thus, the half-life period of the radioactive element, if $75$% of it disintegrates in $40$ min, is $\frac{40}{2}=20$ Minutes.