a =numarul initial de pasageri /=supra
50%din a coboara 1statie 50a/100=a/2
6 urca,25%coboara 3 statie
25/100(a-a/2+6)
1/4(2a/2-a/2+12/2)=1/4(a+12/2)=a+12/8 stim ca raman 21
a-a/2+6-a+2/8=21 inmultim cu 8
8a-4a+48-a-12=168
3a=168-36
a=132:3 a=44
initial au fost 44 pasageri
