Question 1163146
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One question per post -- it is the clear rule of the post.  Re-post the other two problems separately.<br>
For a standard algebraic solution to the problem, the setup would be something like this:<br>
x amount at 7%, plus (4000-x) amount at 9%, equals 300<br>
{{{.07(x)+.09(4000-x) = 300}}}<br>
I leave it to you to finish solving the problem by that method.<br>
Here is a quick and easy method for solving a "mixture" problem like this if a formal algebraic solution is not required.<br>
(1) All $4000 invested at 7% would yield $280 interest; all at 9% would yield $360 interest.
(2) The actual interest amount, $300, is 20/80 = 1/4 of the way from $280 to $360; therefore, 1/4 of the total was invested at the higher rate.<br>
ANSWER: 1/4 of $4000, or $1000, at 9%; the other $3000 at 7%.<br>
CHECK: .09(1000)+.07(3000) = 90+210 = 300<br>