XI. Solve. (Round to the nearest tenth of a percent.) 18. The half-life of radioactive Mathium is 21.\")? years. What percent of a present amount will remain after 18 years? {Use A = C. ext.) 19. Round to the nearest penny. Suppose that $15,000 is invested at an interest rate of 7.5%. How much is the investment worth after 30 months if the interest is compounded: a) quarterly b) continuously