How much of a radioactive kind of barium will be left after 39 days if you start with 52,256 grams and the half-life is 13 days?

Answer

6,532 grams

Q: How many half-life periods have occurred in 39 days? A: 39 ÷ 13 = 3
Q: What fraction of the barium remains after one half-life period? A: ½
Q: What fraction of the barium remains after three half-life periods? A: ½ × ½ × ½ = 1/8
Q: How much barium remains after three half-life periods? A: 52,256 g × 1/8 = 6,532 g