Susan invests
3

times as much money at
11
%
as she does at
7%
. if her total interest after
1
year is
$2000
, how much does she have invested at each rate?

Let's say she invested at 7% "a" amount
now, she invested at 11% three times as much, so 3*a or "3a"

how much is 7% of a? well, (7/100)*a or 0.07a
how much is 11% of 3a? well (11/100)* 3a or 0.11(3a)

now, we know her total yield in interest is 2000 bucks, so whatever 0.07a is and 0.11(3a) are, they add up to 2000

0.07a + 0.11(3a) = 2000

solve for "a".