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