Susan charges twice as much money on credit card with a 7.5% annual interest rate as she does on a card with 6% interest. If her total interest after 1 year is $840, how much did she charge on each card?

Answer 1

credit cards with 7.5% and 6% interest rates? Must be some alternate universe.

Anyway...
amount charged at 6% card --- x
amount charged at other card -- 2x

.06x + .075(2x) = 840
.06x + .15x = 840
.21x=840
x = 4000
She charges $4000 at the 6% card
and $8000 at the 7.5% card