Cardinal number, more commonly called cardinality, is the number of members of a given set.
For example, if P={1,4,6}, cardinality is 3 because there are three member in the set.
For A, the numbers run consecutively from 102 to 155, so there are (155-102)+1=54 members. The cardinality is therefore 54
Similarly, for B={102,...252}, there are (252-102)+1=151 members. So the cardinality is 151.