the cardinal mumber in the set A: 5,10,15.....55

Question

I cant find the set of cardinal number set A:5,10,15,55,,,,,,,please help..My teacher is hard to understand but he was better than my last one.

Answer 1

The cardinal number is the number of objects in the set.  The set A consists of the numbers {5,10,15,...,55}.  Note this is the same as {5(1), 5(2), 5(3),...,5(11)}.  Can you see there are 11 objects in this set?

Answer 2

The cardinal number of a finite set is the number of distinct elements in the set. For example, the set {1, 2, 3} has three distinct elements, so its cardinal number is 3. The set {1, 2, 2, 3} has four elements but only three distinct elements (1,2,3) since 2 is repeated; so its cardinal number is also 3.  So count the number of elements in set A, then subtract one each time an element is repeated (if any).