Question 1195905
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Here is an informal method for solving this kind of problem that is fast and easy if the numbers are "nice".<br>
(1) All $9000 invested at 5% would earn $450 interest; all at 15% would earn $1350 interest.
(2) Perhaps using a number line, look where the actual interest of $520 lies between $450 and $1350: 450 to 520 is a difference of 70; 450 to 1350 is a difference of 900; 520 is 70/900 = 7/90 of the way from 450 to 1350.
(3) That means 7/90 of the total was invested at the higher rate.<br>
ANSWER: 7/90 of $9000, or $700, was invested at 15%; the other $8300 at 5%.<br>
CHECK: .05(8300)+.15(700) = 415+105 = 520<br>