help me bang lupa heheheh..

Putra = L orang => rata-rata = 75
Putri = W orang => rata-rata = 70
Rata-rata keseluruhan = 72
W = 6 + L

Rata-rata = (75L + 70W)/(L + W)
72 = (75L + 70(6 + L)) / (L + (6 + L))
72 = (75L + 420 + 70L) / (2L + 6)
72(2L + 6) = (145L + 420)
144L + 432 = 145L + 420
432 - 420 = 145L - 144L
12 = L

W = 6 + L = 6 + 12 = 18

Banyaknya siswa = L + W = 12 + 18 = 30