Carbon 14 has a half life of 5715 years if an original 100g of 14c in how many years will 25g remain

In this question (t½) of C-14 is 5730 years, which means that after 5730 years half of the sample would have decayed and half would be left as it is. After 5730 years (first half life) 70 / 2 = 35 mg decays and 35 g remains left. After another 5730 years (two half lives or 11460 years) 35 / 2 = 17.5mg decays and ... Missing: 25g