kak tolong ya perbandingan

buku sejarah = 2/5
buku sains = 5/5 - 2/5 = 3/5

buku sejarah : buku sains = 2/5 : 3/5
sejarah : sains = 2 : 3

(3-2)/ (3+2) = 15 / a
1/5 = 15/a
a = (15x5) / 1
a = 75 / 1
a = 75
jumlah sejarah + sains = 75

buku sejarah = 2/5 x 75 = 30 eksemplar