The number of girls, g, in a room is four more than two times the number of boys, b. Half of the number of girls in the room minus the number of boys is 2. The equations below represent the situation.

Call g the number of girls and b the number of boys.

These are the equations that represent the situation:

1) g = 4 + 2b

2) g/2 - b = 2

To solve it you can multiply the second by 2:

=> 2 { g/2 - b = 2 }

=> g - 2b = 4

As you can see this equation is equivalent to the first equation, so this system is not determined (it has infinite solutions).