Let's denote the number of regular tickets sold as "x" and the number of senior citizen tickets sold as "y".

Equation 1: x + y = 450 (Total number of tickets sold is 450)
Equation 2: 500x + 400y = 207,500 (Total revenue generated from ticket sales is $207,500)

To solve this system of equations, we can use the method of substitution.

From Equation 1, we can rewrite it as x = 450 - y and substitute it into Equation 2:

500(450 - y) + 400y = 207,500

Simplifying the equation:

225,000 - 500y + 400y = 207,500
225,000 - 100y = 207,500
-100y = -17,500
y = 175

x + 175 = 450
x = 450 - 175
x = 275

Therefore, the theater sold 275 regular tickets and 175 senior citizen tickets.