The​ half-life of radium is 1690 years. If 20 grams are present​ now, how much will be present in 520 ​years?

Answer 1

520/1690 = 0.3077 half lives

fraction remaining = 0.5ⁿ where n = # half lives = 0.3077

fraction remaining = 0.5^0.3077

^{fraction remaining = 0.8079}

^{grams remaining = 0.8079 x 20 g = 16.16 g = 16 g (to 2 sig figs)}