Radon-222 has a half-life of 3.82 days.

Question

How much of a 100-g sample of radon-222 will remain unchanged after 7.64 days?

75 g

25 g

50 g

100 g

Answer 1

25 g

After 7.64 days, two half-lives have passed. This means that half of the remaining radon-222 will decay in each half-life.

After the first half-life: 100 g -> 50 g
After the second half-life: 50 g -> 25 g

So, 25 g of the 100 g sample will remain unchanged after 7.64 days.