Andy and Ben are friends. One day, they decided to get together, so they started riding bikes towards each other. They plan to meet at the RC Supermarket. Each is riding at 6 km/h. They live 72 km apart. One of them has a pet messenger pigeon and it starts flying when the friends start biking. The pigeon flies back and forth at 18 km/h between the 2 friends, carrying messages until the friends meet.
How many kilometres have the pigeon travelled when the Andy and Ben meet?