Question 1112018
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The interest generated by the 1-st investment is 0.04*10000 = 400 dollars.


She wants  to get at least  700 - 400 = 300 dollars more.


Hence, the second investment of 18000-10000 = 8000 dollars should generate at least 300 dollars interest:


x*8000 >= 300  ====>  x >= {{{300/8000}}} = 0.0375.


<U>Answer</U>.  She must invest the second investment at least  3.75%.
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