Which best describes the error in finding the simple interest earned on $500 at 6% for 18 months?

A nickel is 5 cents or $0.05 and quarters are 25 cents or $0.25. The system of linear equation that would best represent the given scenario in this item is,
                                        x + y = 16
                                        0.05x + 0.25y = 2.20
The values of x and y in the equations are 9 and 7. Thus, there are 9 nickels in his pocket.