Answer :
Explanation:-
Given sets are A = { 1,20,30}
B = { 4,6}
Now,
A-B = { 1,20,30} -{ 4,6}
⇛ A-B = { }
Since there are no common elements in both A and B sets
⇛Number of elements in A-B = 0
⇛n(A-B) = 0
Therefore, n(A-B) = 0
ADDITIONAL COMMENT:
Points to know:-
Empty set :-
- A set having no element is called an empty set or void set or null set .It is denoted by phi or { }
- The cardinal number of the Null set = 0
Difference of two sets :-
- A and B are two sets ,The set of elements which belongs to A and does not belong to B is called The difference between the two sets A and B and it is denoted by A-B.
Cardinal number of a set :-
- The number of all distinct elements in a set is called its Cardinal number .
- It is denoted by n(A).