The half life of an isotope is the amount of time it will take for the given isotope to decay till the point where it reaches half of what amount you had started with.
If the half life of Ra-229 is 4 minutes then every 4 minutes the sample is reduced to half of what you had before
4 minutes.) 50/2 Ra-229 --> 25g Ra-229
8 minutes.) 25/2 Ra-229 --> 12.5 Ra-229
12 minutes.) 12.5/2 Ra-229 --> 6.25 Ra-229 remaining
I beleive this is the correct answer