The half-life of radium is 1620 years. If a laboratory has 12 grams of radium, how long will it take before it has 4 grams of radium left?
4= 12 (1 - .5)^n n = # half lives
1/3 = .5n
n= 1.585
1.585 * 1620 =2567.64 years
+36916 
