Correct Answer : D
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 lies are required
An intelligence agency forms a code of two distinct digits selected from 0,1,2,…….,9 such that the first digit of the code is nonzero. The code, handwritten on a slip, can however potentially create confusion when and read upside down – for example, the code 91 may appear as 16. how many codes are there for which no such confusion can arise
Four brothers and three sisters sit in a single row, facing the photographer’s camera. If the three sisters always sit together, how many different photographs, having all of them, can be eliked?