a sample of radioactive iodine (half life=3 days) originally weighed 10 g, but now weighs 0.625 g. How old is the sample?

Answer 1

0.625 = 10 (1/2)^(t / 3)

log(0.625 / 10) = (t / 3) log(1/2)

or ... 10 , 5 , 5/2 , 5/4 , 5/8

Answer 2

0.625 = 10 * (1/2)^n
0.00625 = .5^n
log 0.00625 = n log 0.5
n = log 0.00625 / log 0.5
n is the number of 3 day periods
time in hours = n * 3 * 24