.
Let x be the 6%-investment amount.
Then
0.04*(1200-x) - 0.06*x = 13
is your equation. Solve it for x.
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If you want to learn more about the method, look into the lesson
- Using systems of equations to solve problems on investment
in this site.
Also, you have this free of charge online textbook in ALGEBRA-I in this site
- ALGEBRA-I - YOUR ONLINE TEXTBOOK.
The referred lesson is the part of this online textbook under the topic "Systems of two linear equations in two unknowns".
Save the link to this online textbook together with its description
Free of charge online textbook in ALGEBRA-I
https://www.algebra.com/algebra/homework/quadratic/lessons/ALGEBRA-I-YOUR-ONLINE-TEXTBOOK.lesson
to your archive and use it when it is needed.