Let us say that:
x = invested at 7%
y = invested at 8.5%
and that:
x + y = 12,000
We can also say that:
0.07 x + 0.085 y = 900
Therefore:
0.07 x + 0.085 (12000 – x) = 900
0.07 x + 1020 – 0.085 x = 900
- 0.015 x = -120
x = $8000
so y is:
y = 12000 – 8000
y = $4,000
$8000 is invested at 7% annual interest while $4000 is invested at 8.5%