.
Let x = amount she invested in Fund B, in dollars (the value under the question).
The interest from Fund A is 0.1*4000 dollars.
The interest from Fund B is 0.03*x dollars.
Total interest is 0.05*(4000+x).
Your equation is
interest + interest = total interest, or
0.1*4000 + 0.03*x = 0.05*(4000+x) dollars.
400 + 0.03x = 200 + 0.05x
400 - 200 = 0.05x - 0.03x
200 = 0.02x ====> x = = 10000.
Answer. $10000 were invested in Fund B.
Check. 0.1*4000 + 0.03*10000 = 700 dollars.
0.05*(4000+10000) = 700 dollars. ! Correct !
-----------------
It is a typical and standard problem on investment.
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.