Sarah invested $ 15000. She put part of it in a term deposit that paid 4% per anum, and the remainder in a treasury bill that paid 5% anum. After one year, the total interest was $690. How much did Sarah invest at each rate?
If there were $x at 4%, then the rest (15000-x) was at 5%.
So add up the interest.
.04x + .05(15000-x) = 690
Find x and then you are almost done.