SOLUTION: One year ago Walt invested $12000. He invested part of the money at 7% and the rest at 9%. He made a total of $970 in interest. How much was invested at 7%? I do not know where

Question 198745: One year ago Walt invested $12000. He invested part of the money at 7% and the rest at 9%. He made a total of $970 in interest. How much was invested at 7%?
I do not know where to place what number in the formula equation
ax2 + bx + c = 0
Found 2 solutions by checkley75, jim_thompson5910:
Answer by checkley75(3666) (Show Source): You can put this solution on YOUR website!
ax2 + bx + c = 0
This is not the correct formula for this type of problem.
.09x+.07(12,000-x)=970 is the correct formula.
.09x+840-.07x=970
.02x=970-840
.02x=130
x=130/.02
x=6,500 amount invested @ 9%.
12,000-6,500=5,500 amount invested @ 7%.
Proof:
.09*6,500+.07*5,500=970
585+385=970
970=970
Answer by jim_thompson5910(35256) (Show Source): You can put this solution on YOUR website!
Note: you do NOT use the quadratic formula here...

Let x=Amount he invested at 7% and y=Amount he invested at 9%

Since he has a total of $12,000, this means that

Also, since he invested some at 7% and the rest at 9%, and received $970, this tells us that

Multiply both sides by 100 to make every number whole.

So we have the system of equations:

Multiply the both sides of the first equation by -7.

Distribute and multiply.

So we have the new system of equations:

Now add the equations together. You can do this by simply adding the two left sides and the two right sides separately like this:

Group like terms.

Combine like terms.

Simplify.

Divide both sides by to isolate .

Reduce.

------------------------------------------------------------------

Now go back to the first equation.

Plug in .

Multiply.

Add to both sides.

Combine like terms on the right side.

Divide both sides by to isolate .

Reduce.

So our answers are and .

This means that he invested $5,500 at 7% and $6,500 at 9%

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