Number problem with watching a magic show

140 children were watching a magic show.  The number of boys was 20 more than twice the number of girls watching the show. How many girls were there?

x = number of girls
2x + 20 = number of boys {number of boys was 20 more than twice the number of girls} 

x + 2x + 20 = 140 {total number of boys and girls was 140}
3x + 20 = 140 {combined like terms}
3x = 120 {subtracted 20 from each side}
x = 40 {divided each side by 3} 

there were 40 girls

- Algebra House