At Longbridge middle school, 240 students said that they are an only child, which is 48% of the schools student enrollment. how many students attend Longbridge middle school?

240 students = 48% of total enrollment

48% means 0.48
"of" means "times" .

240 students = 0.48 x total enrollment

Divide each side by 0.48 :

Total enrollment = 240 students / 0.48 = 500 students