What is the radioactivity of the sample of sodium -24 after 30h?

25%

Explanation:

Sodium-24 is a radioactive isotope of Sodium, which has a half-life of 14.96 hours and undergoes β-decay and changes to a stable isotope Magnesium-24.

Now, considering the half-life of Sodium-24 for approximately 15 hours.

For the first half-life, it will decay 50% of its initial composition in the first 15 hours. So it will contain the remaining 50% of its amount.

Next, for the second half-life, it will decay another 50% of its composition in the next 15 hours. So the remaining composition will be half of 50% i.e. 25%.

Thus, after 30 hours, undergoing 2 half-lives, the remaining composition left is 25% of Sodium-24.