well, in short, the students and chaperones are at a 2 : 9 ratio, and we know there are 171 students, how many chaperones then, so long they maintain a 2:9 ratio?
[tex]\bf \cfrac{chaperones}{students}\qquad 2:9\qquad \cfrac{2}{9}\qquad \qquad \cfrac{x}{171}=\cfrac{2}{9}\implies 9x=2(171) \\\\\\ 9x=342\implies x=\cfrac{342}{9}\implies x=38[/tex]
