Suppose a radioactive substance decays according to the equation $y=2000e^{-0.75t}$, where $y$ represent the mass in grams after $t$ hours.
Assume that the half-life of radium is $1600$ years. If $y_{0}$ milligrams of radium are initially present, write the formula for the number of milligrams $y$ present after $t$ years.
