ini gimana caranya kak ?

diket : X1 = 80
X2 = 75
Xgab = 78
n1 + n2 = 30
n2 = (30 - n1)
ditanya : n2 ( banyak siswa putri )..?
jwb : Xgab = X1 . n1 + X2 . n2
------------------------
n1 + n2
78 = 80 . n1 + 75 .( 30-n1 )
----------------------------
30
78×30 = 80n1 + 2250 - 75n1
2340 = 80n1 - 75n1 + 2250
2340-2250 = 5n1
90 = 5n1
n1 = 18
jawabanya:( D )

maaf kalau salah:)