Answer:
16 days = 16/4 = 4 half-lives
(1/2)^4 = 1/16
1/16 of sample is left after 16 days
6 mg * 16 = 96 mg = amount initially present
4 wks = 4 * 7 = 28 days
28 days / 4 days = 7 half-lives
1/2^7 = .007812
.007812 * 96 mg = .75 mg left after 4 weeks