The half-life of radium is 1620 years. If a laboratory has 20 grams of radium, how long will it take before it has 8 grams of radium left?
8 = 20 (1-.5)t Where t = # of half lifes
t = 1.3219 half lifes
1.3219 x 1620 = 2141.5 yrs
+37163 
