Originally Posted by charps43
the bad news first: I can't give you a complete answer to your problem.
I've attached a drawing of the problem as I understand it.
If is the amount of lead, which is left after a time t, then .
And if is the amount of bismuth, which is left after a time t, then .
(Note: I've to split the following equation - it's too big)
(I suppose that this equation isn't correct, it looks somehow ill to me. But I cann't find my mistake)
For all three amounts of material the equation is true. (Probably the mass isn't constant, because you can observe a loss of material during the process of decay)
And now I have reached the point where I believe that I have made somewhere a severe mistake, because I can't imagine what you should calculate exactly.
But nevertheless I send you this text because maybe you can use it as a hint how or where to start.