Mrs. jackson has $7,000 to invest. if she invests part at 6% simple annual interest and part at 8% simple annual interest, she will get an annual return of $520. how much should she invest at 8%?

Let's say she invest the amounts of "a" at 6% and "b" at 8%.

well, she has to invest on both amounts, 7000 total, thus a + b = 7000

how much is 6% of a? well, (6/100) * a, or 0.06a.
how much is 8% of b? well, (8/100) * b, or 0.08b.

now, we know the annual return in interest from those two amounts is 520.
thus 0.06a + 0.08b = 520



solve for "b".