23a
Let y be the total number at the dance who AREN'T freshman...let x be the number of freshmen
So...initially
x / ( x + y) = .40 cross-multiply
x = .40x + .40y
x - .40x =.40y
.60x = .40y
y = (3/2)x
After 15 more freshmen arrive, we have that
(x + 15) / ( x + (3/2)x + 15) = .52
(x + 15) / ( 5/2 x + 15) = .52 cross-multiply
x + 15 = .52 ( 5/2x + 15)
x + 15 = 1.3x + 7.8
1.3x - x = 15 - 7.8
,3x = 7.2
3x = 72
x = 24 = number of original freshmen
24(3/2) = 36 = number not freshmen
Proof
Initially there were 24 + (3/2)24) = 60 students at the dance
(24) / 60 = ,40 = 40%
After 15 more freshmen arrive.....there are 60 + 15 = 75 students
(24 + 15) / ( 60 + 15) = 39 / 75 = .52 = 52%