An executive invests $28,000, some at 8% and the rest at 5% annual interest. If he receives an annual return of $1,760, how much is invested at each rate?
at 8% interest $
at 5% interest
s + r = 28000
.08 s + .05 r = 1760 ... multiplying by 20 ... 1.6s + r = 35200
subtracting equations (to eliminate r) ... 0.6 s = 7200
solve for s , then substitute back to find r