text: boys=x\ngirls=2x-20\n\nadd them together:\n\nx+2x-20=250\n\nCombine like terms:\n\n3x-20=250\n\nadd 20 to both sides:\n\n3x=270\n\ndivide both sides by 3:\n\nx=90\n\nsubtract 90 from 250 to get the number of girls, which is 160\n\nHope this helps!