Also, by this simple method:

0.1 = 2^-(500,000/h)

1/10 = 2^-(500,000/h) take the reciprocal of both sides,

10 = 2^(500,000/h) take the log of both sides

500,000/h =log(10) / log(2)

500,000/h =3.321928...

h =500,000 / 3.321928

h ~ 150,515 years - the half-life of this element.