Hi there!
So let's start by writing down everything we know about the situation in terms of equations. The stadium has three sections, A, B, and C, which in total is 55,000 seats. We can write that as:
A + B + C = 55,000 (Eq. 1)
The number of seats in Section A equals the total number of seats in Sections B and C. Therefore:
A = B + C (Eq. 2)
Lastly, in a sold-out show (where all seats are purchased), the total revenue is $1,448,500. Seats sell for $35 in Section A, $20 in Section B, and $15 in Section C. We can write this as:
35A + 20B + 15C = 1,448,500 (Eq. 3)
We have three equations written, with three unknowns. This is known as a system of equations, and one way of working through this is by the substitution method. To substitute, we can arrange the equations in terms of one variable, and plug it into the other equations.
Let's start with Eq. 2:
A = B + C
It already defines A in terms of B and C, so this is a good place to begin. We'll move to Eq. 1 and replace every A with its equivalent, B + C:
A + B + C = 55,000
(B + C) + B + C = 55,000
2B + 2C = 55,000
Now we'll arrange this equation to define B in terms of C:
2B + 2C = 55,000
2B = 55,000 - 2C
B = 27,500 - C (Eq. 4)
Our last step of substitution is to plug Eq. 4 into Eq. 3, the last equation of our system. So for every A, we'll replace it with B + C. Then, for every B, we'll substitute it with 27,500 - C:
35A + 20B + 15C = 1,448,500
35(B + C) + 20B + 15C = 1,448,500
35[(27,500 - C) + C] + 20(27,500 - C) + 15C = 1,448,500
962,500 + 550,000 - 20C + 15C = 1,448,500
-5C = -64,000
C = 12,800 seats
Now we have the number of seats in Section C. We can plug this into Eq. 4 to solve for B:
B = 27,500 - C
B = 27,500 - 12,800
B = 14,700 seats
Lastly, we can plug our values for B and C into either Eq. 1, 2, or 3 to find A. Let's do Eq. 2:
A = B + C
A = 12,800 + 14,700
A = 27,500 seats
Hope that helps!