If a radioactive isotope has half life of 5 days,how long will it take for its radioactive strength to be a maximum of one thousandth,(1/1000) of its original value?

Answer 1

1/2 = e^-5k
ln .5 = -5 k
k = .1386

so
x = Xo e^-.1386 t

.001 = e^-.1386 t

-6.91 = -.1386 t
t = 49.8 days

check
about 10 five day periods
.5^10 = .000976
close enough to .001