Question

How long will it take for half of Bismuth-214 to decay to lead? (Refer to the Uranium-238 decay chain series.)

Answer 1

Yeah, tough. You should be able to see that Bi-214 doesn't decay directly to lead,
but instead has 20 minute half-life to Po-214.
So, how long does it take half the Bi to become Po?
next, the Po-214 does alpha decay almost immediately
(1/6 millisec) after it becomes Po.