Fie f-varsta fiului si t-varsta tatalui;_____ __t=f+19_________________________ __t+10=2•(f+10)_=>_t+10=2•f+20______ __t=2•f+20-10=2•f+10_______________ __obs:t=f+19 si t=2•f+10_____________ __=>_f+19=2•f+10_=>_2•f-f=19-10_____ __f=9,iar t=f+19=9+19=28