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Home Q & A Board Science-Related Homework Help Mathematics
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SmokeyV4L SmokeyV4L
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11 years ago
If you invested $12,000, part of it at 7% and the rest at 9% and made $970 in interest. How much was invested at 7%?  I do you put that in a quadratic equation and solve it out?
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11 years ago
I assume the investment is for one year, and that the interest is paid yearly.

Let x denote the amount (in dollars) invested at 7%
Then (12000 - x) is the amount invested at 9%

The investment at 7% pays 0.07x dollars interest
The investment at 9% pays 0.09(12000 - x) dollars interest
These two add to 970:

0.07x + 0.09(12000 - x) = 970
0.07x + 1080 - 0.09x = 970
-0.02x = -110
x = -110/-0.02 = 5500

Therefore, $5500 was invested at 7% and $6500 was invested at 9%.

BTW, it's linear, not quadratic.
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