The mass of radioactive material present, m, in a sample of initial mass, mo, is given by the equation: \frac{d m}{d t}=-k \cdot m Obtain an equation for the decay

of reactive material. If the half-life, t½, of the material is 54.5 years, find the proportion of material (i.e m/mo) left after 90years. Give your answer to three significant figures.