Notam cu f - numarul fetelor si cu b - numarul baietilor. [tex]\\ f=2*b \\ b+4=f-5 \\ \left \{ {{f=2*b} \atop {b+4=f-5}} \right. \\ \left \{ {{f=2*b} \atop {b=f-9}} \right. \\ \left \{ {{f=2*(f-9)} \atop {b=f-9}} \right. \\ \left \{ {{f=2*f-18} \atop {b=f-9}} \right. \\ \left \{ {{-f=-18|*(-1)} \atop {b=f-9}} \right. \\\left \{ {{f=18} \atop {b=f-9}} \right. \\\left \{ {{f=18} \atop {b=18-9}} \right. \\\left \{ {{f=18} \atop {b=9}} \right. [/tex]