The half-life of radon-222 is about four days. After how many days is the amount of radon-222 equal to one-sixteenth of its original amount?

I know the answer is 16,but why?

Please steps...thank you!

1 answer

0 days = 1 gram. Start with 1 gram.
4 dais = 1/2 g
8 days = 1/4 g
12 days = 1/8 g
16days = 1/16 g