Susan invests twice as much money at 7.5% as she does at 6%. if her total interest after a year is $840 how much does she have invested at each rate?

Let x be the amount invested at 6%

840 = 0.06x + 0.075(2x)
840 = 0.06x + 0.15x
840 = 0.21x
x = 840/0.21
x = 4000

Therefore, she invests $4000 at 6% and $8000 at 7.5%

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step-by-step explanation:

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x² + 3x + 7 = 0       [ax² + bx + c]

a = 1

b = 3

c = 7

quadratic formula:

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plug in the numbers into the variables

your answer is a