Investing Find the final amount (rounded to the nearest dollar) in each of these retirement accounts, in which the rate of return on the account and the regular contribution change over time. (See Example 3.) 9. $400 per month invested at 4%, compounded monthly, for 10 years; then $600 per month invested at 6%, compounded monthly, for 10 years. 12. $1500 per quarter invested at 7.4%, compounded quarterly, for 15 years; then $1000 per quarter invested at 4.2%, compounded quarterly, for 10 years. (Compare with Exercise 11.)