It takes 6400 years for one gram of radium to decay away to only 1/16-one- sixteenth-of a gram. The half- life of radium is

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The half-life is the time it takes for a substance to reduce to half its initial amount. If 1 gram decays to 1/16 of a gram in 6400 years, it means it has gone through 4 half-lives (since 1/16 = (1/2)^4). Therefore, the half-life is 6400 years / 4 = 1600 years.

It Takes 6400 Years for One Gram of Radium to Decay