Let A and B be the number of advance and same-day tickets sold. From the question we can write the following relations:
[tex]\begin{gathered} A+B=70 \\ A\times15+B\times35=1650 \end{gathered}[/tex]
From this, we can isolate A in the first relation and substitute it in the second. This way we can find the value of B, as follows:
[tex]\begin{gathered} A=70-B \\ \\ (70-B)\times15+35B=1650\to1050-15B+35B=1650\to \\ \to20B=1650-1050=600\to B=\frac{600}{20}=30 \end{gathered}[/tex]
From this, we can substitute the value of B in the first relation to find A.
[tex]A+30=70\to A=70-30=40[/tex]
From this, we conclude that:
The number of advance tickets sold is 40
The number of same-day tickets sold is 30
