Question 155060: Walt made an extra $9000 last year from a part-time job. He invested part of the money at 9% and the rest at 8%. He made a total of $770 in interest. How much was invested at 8%?
Answer by jim_thompson5910(35256)   (Show Source):
You can put this solution on YOUR website! Let x = amount invested at 9% and y = amount invested at 8%
Since Walt only invested $9,000, this means that the total (ie sum) of both accounts add to $9,000. Algebraically, this means that 
 Start with the simple interest formula. Note: "P" is the amount invested, "r" is the interest rate, and "t" is the time.
 Plug in  and  and  (since this only occured last year)
So the interest formula for the 9% account is 
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Go back to the simple interest formula.
 Plug in  and  and
So the interest formula for the 8% account is 
Since we know that the total interest adds to $770, this means that 
 Multiply every term by 100 to make every number a whole number
So we have the system of equations:
Start with the first equation
 Subtract "x" from both sides
Move onto the second equation
 Plug in
 Distribute
 Combine like terms.
 Subtract 72,000 from both sides
So Walt invested $5,000 at 9%.
Go back to the isolated equation
 Plug in
 Subtract
So Walt invested $4,000 at 8%
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Answer:
So Walt invested $5,000 at 9% and $4,000 at 8%
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