There are many towns on the island of Tetra, all connected by roads. Each town has three roads leading to three other towns, one red road, one blue road, one yellow road, where no two rads meet other than at towns. If wou start from any town and travel along yellow and red roads alternatively (RYRY...) you will get to your starting town after having travelled over six different roads. This is the same as travelling along Yellow and Blue roads (YBYB...). On the other hand, going along red and blue roads alternatively will always get you back to the starting point after travelling along four different roads (RBRB). How many towns are there on Tetra?
