15. So half-life means after 3.8 days, this amount is halved.
After 3.8 days: 99.3
After 7.6 days: 49.65
After 11.4 days: 24. 825
After 15.2 days: 12.4125
After 19 days: 6.20625
After 22.8 days: 3.103125
So in about 23 days, you'd have about 3.1 grams left.
Don't know about the rest, but I hope I helped on this one :)