The average earning of a group of persons is Rs. 78 per day. The difference between the highest earning and lowest earning of any two persons of the group is Rs. 38. If these two people are excluded the average earning of the group decreases by Rs. 2. If the minimum earning of the person in the group lies between 66 and 70 and the number of persons in the group was equal to a prime number. The number of persons in the group initially was: