The half-life of radium-226 is 1620 years. How much of one gram of radium-226 will remain unchanged after four half-lives?

After 4 half-lives, the amount remaining is ( (1/2)^4 ), which equals 1/16. Therefore, 1 gram of radium-226 will have 1/16 gram unchanged after four half-lives, which is 0.0625 grams.