There are two equations you can get from this.
2.5X+1.25Y=1375
X+Y=700
By solving for X, we can then plug the 2nd equation into the first equation.
X=700-Y
2.5(700-Y)+1.25Y=1375
1750-2.5Y+1.25Y=1375
375=1.25Y
Y=300
After we get the Y, we solve for the X.
300+X=700
X=400
There were 400 adults, and 300 children. A quick check proves this.
2.5(400)+1.25(300)=1375