Remove the "return" keyword from all the "if" conditions.
public double taxErgazomenos()
{
if(getVasikosMisthos()<=12000) {
foros=0.01*getVasikosMisthos();
}
else if(getVasikosMisthos()>=12000 && getVasikosMisthos()<=16000)
{
foros = 15 / 100 * getVasikosMisthos();
}
else if(getVasikosMisthos>=16000 && getVasikosMisthos()<=18500)
{
foros=20/100*getVasikosMisthos();
}
else if(getVasikosMisthos()>=18500 &&getVasikosMisthos()<=24000)
{
foros=28/100*(getVasikosMisthos());
}
else if(getVasikosMisthos()>=24000 &&getVasikosMisthos()<=28000)
{
foros=35/100*getVasikosMisthos();
}
else if(getVasikosMisthos()>=28000 &&getVasikosMisthos()<=36000)
{
foros=40/100*getVasikosMisthos();
}
return foros;
}