In the late nineteenth century, the scientist Marie Curie performed experiments that led to the discovery of radioactive substances. A radioactive substance is a substance that gives off radiation as it decays. Scientists describe the rate at which a radioactive substance decays as its half-life. The half-life of a substance is the amount of time it takes for one-half of the substance to decay. Radium has a half-life of 1600 years. How much radium will be left from a 1000-gram sample after 1600 years?