Let m be the percentage mass of radium present at time t.
Let be the mass present at time .
where k is a real constant (k is actually negative since this is "exponential decay". You can write k=-b or something to show that you know it is negative. I just left it because it doesn't change the overall answer at the end.)
using the intial conditions:
when t=I years:
At t=2I years:
and I think we're done! =D
P.S: I did change notation. I used m instead of p. If you like you can work it through again with m=p or just write this at the end.