Question 1159372
Solve the following word problem using matrices:

Part of $30,000 is invested at 6% yearly interest, another part at 7%, and the remainder at 8% yearly interest.  The total yearly interest income from the three investments adds up to $2200.  The sum of the amounts invested at 6% and 7% equals the amount of money invested at 8%.  How much is invested at each rate? 
<pre>With the amount invested at 8% being the total of the 6% and 7% investments, it goes without saying that $15,000 was invested in the 6% and 7%, and $15,000 was invested at 8%.
Let the amount invested at 6% be S
Then amount invested at 7% = 15,000 - S
We then get the following EARNINGS equation: .06S + .07(15,000 - S) + .08(15,000) = 2,200
.06S + 1,050 - .07S + 1,200 = 2,200 
.06S - .07S = 2,200 - 2,250
 - .01S = - 50
Amount invested at 6% = {{{highlight_green(matrix(1,3, (- 50)/(- .01), "=", "$5,000"))}}}
It then follows that $15,000 - $5,000, or $10,000 was invested at 7%.
It was already revealed how much was invested at 8%.
Just realized that this asks for a matrices-solution. The other person did it anyway!