At a party at the middle school over on Third Avenue next to the bank, 60% of the students are seventh graders. These students are joined by 20 more eighth graders, all of whom like to dance. The party is now 58% seventh graders. How many students are now at the party?
+312 Call the original number of students, S...and the number of girls = .60S
Total students after 20 boys arrive = S + 20
Also:
Number of girls divided by total number of students = 58%
.6S / ( S + 20 ) = .58 multiply both sides by S + 20
.6S = .58( S + 20 )
.6S = .58S + 11.6 subtract .58S from both sides
.02S = 11.6 divide both sides by .02
S = 11.6 / .02 = 580
So...the total number of students = 580 + 20 = 600 
