.
1 - the system of 2 equations
F + B = 375, (1) (counting seats)
25*F + 21*B = 8875. (2) (counting money)
To solve it, express F = 375 - B from (1) and substitute into (2). You will get
a single equation for one unknown B:
25*(375 - B) + 21*B = 8875. (3)
Simplify and solve for B.
When you find B, calculate F from (1).
2 - One equation
Let B be the number of the Balcony seats.
Then the number of the Floor seats is (375-B).
The "money" equation is
25*(375 - B) + 21*B = 8875,
exactly as (3).
If you want to see more solved problems of this type, look into the lesson
- Using systems of equations to solve problems on tickets
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.