.
Let x be an amount invested at 5%, in dollars.
Then the amount invested at 8% is the rest (12000-x) dollars.
Your basic equation is THIS:
interest + interest = total interest
0.05*x + 0.08*(12000-x) = 840.
0.05x + 0.08*12000 - 0.08x = 840
-0.03x = 840 - 0.08*12000 = -120 ====> x = = 4000.
Answer. $4000 was invested at 5% and the rest 12000 - 4000 = 8000 dollars was invested at 8%.
Check. 0.05*4000 + 0.08*8000 = 840 dollars. ! Correct !
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To see many other similar solved problems on investment, look into the lesson
- Using systems of equations to solve problems on investment
in this site.
You will find there different approaches (using one equation or a system of two equations in two unknowns), as well as
different methods of solution to the equations (Substitution, Elimination).
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.