18/2 does not work because Roger did not invite the same number of girls as boys. Dividing 18 guests by 2 simply splits the guests evenly in half: 9 boys and 9 girls. The number of boys needs to be twice the number of girls.
That's three parts total, that make up the 18 guests.
So, divide 18 by three, to get the number of people in each part.
there will be 6 girls and 12 boys!
