x nr pasageri initial
50x/100=x/2 =au coborat la statia 1
x-x/2=x/2 =pasageri dupa statia 1
x/2+6=pasageri dupa statia 2
(x/2+6)*25/100=(x/2+6)*1/4=x/8+3/2=pasageri care au coborat la statia 3
x/2+6-(x/8+3/2)=21
x/2+6-x/8-3/2=21
4x+48-x-12=168
3x=132
x=44 pasageri inainte de prima statie