A store sell mints for 50美分each or 4.8美元for a case of 12 min

A store sell mints for 50美分each or 4.8美元for a case of 12 mints.The cost per mint is what percent greater when the mints are purchased separately than when purchased in a case?
第二道：if m is a positive integer,and if 3+16/m is an interger less than 19,which of the following must be true of A:m=19,B:m is even,C:m=16,D:m is a prime number E:m is a multiple of four 感激不尽！