The number of girls in Mrs. Manning’s classes was 12 less than twice the number of boys. She had a total of 108 students in her classes. How many girls and how many boys are in Mrs. Manning’s classes?

40 boys and 68 girls

let's start with variables and an equation.

boys: x

girls: 2x - 12

and we know that boys and girls make up the class. so let's add the two expressions.

x + 2x - 12 = 108

3x - 12 = 108

    + 12     + 12

3x = 120

x = 40 boys

BUT x is only the number of BOYS in the class. so we have to find girls now. let's plug in our value of x into the girls' equation.

2x - 12 = # of girls

2(40) -12 = # of girls

80 - 12 = # of girls

68 girls

let's check our answer!

68 + 40 does in fact add to 108, therefore our answer is correct.