Via half-life equation we have:
\(A_{final}=A_{initial}(\frac{1}{2})^{\frac{t}{h} }\)
Where the initial amount is 50 grams, half-life is 4 minutes, and time elapsed is 12 minutes. By plugging those values in we get:
\(A_{final}=50(\frac{1}{2})^\frac{12}{4}=50(\frac{1}{2})^{3}=50(\frac{1}{8})=6.25g\)
There is 6.25 grams left of Ra-229 after 12 minutes.
