Question

There are 12 towns grouped into four-zones with three towns per zone. it is intended to connect the towns with telephone lines such that every two towns are connected with three direct lines if they belong to the same zone, and with only one direct line otherwise. how many direct telephone lines are required?

Answer 1

14 lines are requried in which 2 town of same zone will get 3 direct line so the 8 town 2 of different zone wil requrie 12 direct line that is 3 per 2 town and the left out one town from each zone in which we can make pair of town from different zone and extra 2 line will be requried