Let x be the number of adult tickets and let y be the number of children.
We know that in total there were sold 700 tickets, that means:
[tex]x+y=700[/tex]Now we know that each adult ticket was $2 and the children's tickets was $1, and that the total was $900, this means that:
[tex]2x+y=900[/tex]Then we have the following system of equations:
[tex]\begin{gathered} x+y=700 \\ 2x+y=900 \end{gathered}[/tex]To solve it we substract the second equation from the first one, then we have:
[tex]\begin{gathered} x+y-2x-y=700-900 \\ -x=-200 \\ x=200 \end{gathered}[/tex]Now that we have the value of x we plug it in the first equation to find y:
[tex]\begin{gathered} 200+y=700 \\ y=700-200 \\ y=500 \end{gathered}[/tex]Therefore, there were 200 adult's tickets and 500 children's tickets sold.