The unknown x is how much to invest at 10%.
The combined amount being invested is $12,000 + x and you want the combined rate to be 7%
Write that as 1.07 (12,000 + x)
The individual investments are 1.06 (12,000) and 1.10 (x)
Add the individual investments together and set them equal to the combined investment and solve for x
1.06 (12,000) + 1.1 (x) = 1.07 (12,000 + x)
12,720 + 1.1x = 12840 + 1.07x
.03x = 120
x = $4000 is invested at 10%
You can check by plugging back in.
11/19/2013

Susan L.