tolong ya,pakai cara

presentasi pensil
40+15+25-100= 20%

dik = penjualan pulpen = 20
presentasi pulpen = 25%
presentasi pensil = 20%
ditanya = penjualan pensil=?

25/20= 20/x
5/4=20x
5x= 80
x= 80/5
x = 16
penjualan pensil = 16 lusin