Radioactive iodine treatment is so successful at treating hyperthyroidism that it has virtually replaced thyroid surgery. To the nearest full day, determine how long it will take for 400 millicuries of I-131, which has a half-life of 8 days, to decay to 3.125 millicuries.

Question

56 days,
7 days,
128 days,
or
8 days

which one is it?

Answer 1

3.125/400 = (1/2)^(t/8)
(if t is in days)
7.8125*10^-3 = (1/2)^(t/8)

t/8 = Log(7.8125*19^-3)/Log(1/2)
= 7
t = 56 days

Answer 2

thank you so much!!