Bismuth-210 decays at a rate proportional to the amount present. A sample of Bismuth-210 that initially had a mass of 1000 mg decayed 500 mg in 5 days. Let M(t) be the mass of Bismuth-210 in milligrams t days after the initial sample of 1000 mg began to decay. Set up and solve an Initial Value Problem for determining M(t).