SOLUTION: Please help me with this. I've done similar problems when I was just calculating for how much to invest of two different percentages, but having to calculate the total interest is

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Question 1003369: Please help me with this. I've done similar problems when I was just calculating for how much to invest of two different percentages, but having to calculate the total interest is throwing me off:
Wendy was awarded a volleyball scholarship to the University of Michigan, so on graduation her parents gave her the $17,000, they had saved for her college tuition. She opted to invest some money in a privately held company that pays 10% per a year, and evenly split the remaining money between a money market account yielding 2% and a high risk stock that yielded 40%. At the end of the first year she had $19,910 total. How much did she invest in each of the three?
_____ @ 10%
______@ 2%
______@ 40%

Answer by maxitee(11) About Me  (Show Source):
You can put this solution on YOUR website!
The solution is derive an equation and link two variables together. See below.
Let's call the amount invested for the 10%, A
Let's call the 2%, B and the 40% C.
From the question, B = C.
If she made $19,910 at the end of the first year. We try to sum up all the investments that gave her the returns. Using the above A, B and C. We have.
(10% of A plus A)***THIS IS EVERYTHING SHE GETS FROM THE INVESTMENT, i.e we add her profit(10%) plus her initial investment ******
Applying the above for all other investments we have.
%2810A%2F100+%2B+A%29+%2B+%282B%2F100+%2B+B%29+%2B+%2840C%2F100+%2B+C%29+=+%2419910+
Being that B = C, we can substitute B for C. After doing that and simplifying the above equation we have.
%2810A%2B100A+%2B+2B+%2B+100B+%2B+40B+%2B+100B%29%2F100+=+%2419910
Let's call the above equation (i)
Remember that she invested all her money. Therefore if we sum all the invested amounts, A B and C we will equate it to her initial $17,000
A+%2B+B+%2B+C+=+%2417000
Being that B and C are the same we have a new equation from the above as.
A+%2B+2B+=+%2417000
We shall call the above equation (ii).
If we make A subject of formula in equation (ii) we have A = 17,000 - 2B.
So we simply substitute the value of A into equation (i) above. See the result.
110%2817000-2B%29+%2B+242B+=+1991000
From the above B will equate = $5,500
If B is $5,500 C is equally $5,500 which puts A as $6,000.
In conclussion,
$6,000 @ 10%
$5,500 @ 2%
$5,500 @ 40%