Answer:
In order to solve half-life questions, consider the equation ...
AT = A0 2(-T/H)
... where A0 is the initial radioisotope's activity, AT is the decayed activity after time T, and H is the half-life. Given 3 of these parameters, you can solve for the fourth. For instance, given starting activity, ending activity, and time, you can solve for half-life as follows ...
AT = A0 2(-T/H)
AT/A0 = 2(-T/H)
ln2(AT/A0) = -T/H
H = -T/ln2(AT/A0)
... and don't forget that ln2(x) = log(x)/log(2)