Doris invested some money at 7% and some money at 8%.She invested $6000 more than she did at 7%. Her total yearly interest from the two investments was $780. How much did Doris invest at each rate?

Answer 1

If she invested $x at 7%, then if I understand what you wrote, she invested $(x+6000) at 8%, right?
Now add up the interest
.07x + .08(x+6000) = 780