How many persons live between the floors on which F and G live? 

From (i), (ii) and (iv), we will get the basic information as shown below: From (i), A lives on floor no. 3. From (iv), G cannot live on floor no. 4 and 2. So, G lives on floor no. 6. From (v), we will have two cases: D lives either on floor no. 1 or 7. From (ii), F lives on floor no. 2 and B lives on floor no. 4. The one who likes Classmate lives on floor no. 6. From (iii), The one who likes Pilot and Montblanc live on floor no. 5 and 1 respectively. From (iv), C lives on floor no. 7.  Case 2 will get discarded as C does not like neither Pilot nor Montblanc.  From (iii) and (vi), the persons who like Nataraj lives on floor no. 4. The person who likes Apsara lives on floor no. 5. From (i), the person who likes Staedtler lives on floor no. 7. From (v), the one who likes Faber castell and Doms live on floor no. 6 and 3 respectively.   Only pencil left for floor no. 2 is Camlin. From (v), The person who likes Linc and Pelikan lives on floor no. 3 and 2 respectively. Only pen left for floor no. 7 is Parker.  Final arrangement as shown below: