We will create two equations. For he invested in two banks, one paying 4.5% and the other 6%, we have "x" and "y" to represent the two banks such as: Let x be the bank paying 4.5 % Let y be the bank paying 6% X + Y=$65,100 If he gets $231 more on the 6% investment than that on the 4.5% investment X=Y+$231