It is evident that you need to find the solution of the system, if it exists.
Let's work the equations.
The second equation (which corresponds correctly to the second statement) is:
g/2 - b = 2, which you can be transformed in this way:
1) Multiply by 2 => g - 2b = 4
2) Transpose - 2b => g = 4 + 2b.
Now you can see that this equation is the same than the firs equation, which means that the second statement does not provide any new information.
Mathematically, this is a system with infinite solutions, and you cannot determine the number of boys or girls without more information which provides and independent equation.
Answer: there are infinite solutions to the system.
