Question 156486
Ted put $4000 in his savings account at Bank A.
Let interest rate be x ; Interest = 4000x dollars
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He put $6,000 in an account at Bank B that paid interest at twice the rate as Bank A.
Interest rate = 2x ; Interest = 6000x dollars
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HIs total simple interest for the year was $640. 
What was the interest rate at each bank?
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EQUATION:
4000x + 6000x = 640
10000x = 640
x = 0.064 = 6.4% (Bank A interest rate)
2x = 0.128 = 12.8% (Bank B interest rate)
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Cheers,
Stan H.