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Walt made an extra $7000 last year from a part-time job. He invested part of the money at 10% and the rest at 7%. He made a total of $580 in interest. How much was invested at 7%?Select one:

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Trinacheemalapid
We know that
Interest= PNR/ 100.
where P is the principal
R is the rate of Interest
 N is the no of years the sum has been invested for. 
Since N  is not given we assume that to be 1
Therefore we have 
10x /100 + (7000 -x)*7/100= 580

Multiplying both sides by 100
10x+(7000-x)*7 = 58000
simplifying
10x+ 49000 - 7x = 58000
3x = 58000-49000
3x = 9000
x= 3000
 therefore the sum of money invested at 7% = 7000 - 3000 = 4000