How much was invested?

Question

Walt made an extra $8000 last year from a part-time job. He invested part of the money at 9% and the rest at 7%. He made a total of $660 in interest. How much was invested at 7%?

Answer 1

.07x + .09y = 660

x + y = 8000

x=8000-y

.07(8000-y) + .09y =660

560 - .07y +.09y =660

.02y = 100

y = 5000  therefore x = 3000

Answer 2

lets say x = the money he invested at 9% and y = the money he invested at 7%

x + y = 8000

0.9x + 0.7y = 660

solve for y