After 4 half-lives, there was 1 gram.
1 half-life earlier, after 3 half-lives, there must have been 2 grams.
1 half-life earlier, after 2 half-lives, there must have been 4 grams.
1 half-life earlier, after 1 half-life, there must have been 8 grams.
1 half-life earlier, when it started decaying, there must have been 16 grams. (B)
