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%.