Rule 20. The service on a dealer of any notice, summon, order or proceedings under the Act or under these rules, may be effected in any of the following ways, namely :
(a) by delivering or tendering it to such importer or to his manager or to his agent who is concerned with the business ; or 
(b) if such importer or his manager or agent is not found, by leaving it at his last known place of business of residence or by delivering or tendering to some adult member of his family ; or 
(c) if the address of such importer is known to the assessing authority, by sending it to him by registered post with acknowledgement due ; or 
(d) if any or all of the modes aforesaid is not practicable, by affixing it in some conspicuous place at his last known place of business or residence.