Let,
Number of Advance Tickets = a
Number of Same-Day tickets = s
Given,
Total 45 tickets sold
We can write

Also, each advance ticket cost $15 and same day tickets cost $20 for a total of $825. Thus, we can write:

We will multiply the first equation by - 15 and then add both equations. Then, solve for "s". The steps are shown below:

Now, we can use this value of "s" and put it into Equation 1 and find the value of "a". Shown below:

Answer
Number of advance tickets sold: 15
Number of same-day tickets sold: 30