Need help on the following question,

Iodine-123, used in thyroid therapy, has a half-life of 13.27 hours. How many half-lives are required for a 160 mg sample of iodine-123 to decay to 5.0 mg?
A) 1.0
B) 32
C) 5.0
D) 0.031

Question

Need help on the following question,

Iodine-123, used in thyroid therapy, has a half-life of 13.27 hours.  How many half-lives are required for a 160 mg sample of iodine-123 to decay to 5.0 mg?
A) 1.0
B) 32
C) 5.0
D) 0.031

anonymous · Answer

t = 0 => 160 mg
t = 13.27 hrs => 80 mg
t = 13.27 * 2 hrs => 40 mg
t = 13.27 * 3 hrs => 20 mg
t = 13.27 * 4 hrs => 10 mg
t = 13.27 * 5 hrs => 5 mg

So after 66.35 hrs, i.e. after 5 halflives, there will be only 5 mg left.