Susan invests

2

times as much money at

8%

as she does at

4%

. If her total interest after

1

year is

$800

, how much does she have invested at each rate?

Let f=amount invested at 4% and e=amount invested at 8%

0.04f+.08e=800

However, e=2f so the above equation becomes:

0.04f+0.08 (2f) = 800

0.04f+0.16f=800

0.2f=800

f=$4000, since e=2f

e=$8000

So Susan invested $4000 at 4% and $8000 at 8%.