You invested $10,000 in two accounts paying 5% and 7% annual interest, respectively. If the total interest earned for the year was $640, how much was invested at each rate?

Answer 1

640/10000 = 6.4%

6.4% is within .6 of 7% and within 1.4% of 5%

you'll invest 1.4/.6 times as much at 6% as at 5%

1.4/.6 = 7/3

7x/3 + x = 10,000

10x/3 = 10000

x/3 =1000

x =$3000 invested at 5%

7x/3 = $7000 invested at 7%

.07(7000 + .05(3000= 490+ 150= $640

or

.07x + .05(10,000-x) = 640

.02x = 640 - 500= 140

x = $7000

3000(.05 + 7000(.07) = $150 + $490 = $640