Formulate but do not solve the problem. Kelly Fisher invested a total of $20,000 in two municipal bonds that have yields of 8% and 9% interest per year, respectively. If the interest Kelly receives from the bonds in a year is $1680, how much did she invest in each bond? (Let x and y denote the amount, in dollars, invested at 8% and 9%, respectively.)
____ = 20,000 _____ = 1680