Radioactive decay

Radioactice substances decay exponentially. The amount of radium present at an time t obeys the law Q(t)= Qoe^-kt, where Qo. is the initial amount present and k is a suitable positive constant. The half-life of a radioactive substance is the time required for a given amount to be reduced by one-half. Now, it is known that the half-life of radium is approximately 1600 years. Suppose initially there are 200 miligrams of pure radium. Find the amount left after t years. What is the amount left after 800 years?