Question

Investment problem... see attached. I keep coming up with the wrong answer >:(

Answer 1

Hey Amy :)

Notice that this principle amount 21000 is being split into two amounts.
Maybe we'll call these amounts X and Y.
These are the principles for each investment.
The sum of these principle investments gives us the total of 21000.
\[\large\rm X+Y=21000\]

The other equation we can set up is one related to the interest earned from each investment. 5% of X and 8% of Y gives us the 1440 that was earned in a single year. If we convert these percents to decimal values, we can write that relationship like so,\[\large\rm .05X+.08Y=1440\]

Answer 2

If we multiply this second equation by 100,\[\large\rm 5X+8Y=144000\]And the first equation by -5,\[\large\rm -5X-5Y=-105000\]It gives us a system of equations that we can add together, ya?
The way I did it here, your X's will cancel out,
allowing you to first solve for Y, the 8% investment.

Answer 3

Hey thanks @zepdrix that really helped