How long will it take a sample of polonium-210 with a half-life of 140 days to decay to one-sixty-fourth of its original strength?

Answer 1

1/2 = e^k(140)
ln .5 = 140 k
k = .00495
so
1/64 = e^-.00495 t

-4.159 = -.00495 t
t = 840 days

Answer 2

alternatively
1/64 = (1/2)^6
so 6 half lives
=140*6 = 840 days