7, 9, 11, 13, 15 . Let's assume that the house numbers increase by 2 each time. Then the house numbers are X-4, X-2, X, X+2, X+4. The average of this sequence is the middle number = 55 / 5 = 11 The numbers are then 7, 9, 11, 13, 15
There are 6 students to be seated around a circular table. In how many ways they can be seated if two particular persons are next to each other.