Answer: 1) A. (2, 3)
2) B. (-1, -1)
3) 10 children
Step-by-step explanation:
1) 
Since the y is already isolated in the first equation, to solve the system you simply substitute that expression into the second equation and then solve, finding the value of x:

And then you substitute that value into the first equation and solve to find the value of y:

So, the solution of the system is (2, 3).
2) 
Since the y is already isolated in both equations, to solve the system you simply equalize the first and the second expression and then solve, finding the value of x:

And then you substitute that value into the first or the second equation (whichever you like) and solve to find the value of y:

So, the solution of the system is (-1, -1).
3) 
To solve the system, the easiest way is to isolate the y in the first equation and then substitute the expression obtained into the second equation, finding the value of x.
From the first equation: 
Substituting:

And then you substitute that value into the first equation and find the value of y:

So, there were 10 children in the group.