Final answer:
The cardinal number for the set {1,2,3,4,5,4,3,2,1} is 5, as there are 5 distinct elements after ignoring duplicates.
Step-by-step explanation:
To determine the cardinal number of the given set, {1,2,3,4,5,4,3,2,1}, we count the number of distinct elements in the set. A set does not include duplicates, so each element is only counted once, regardless of how many times it is listed.
The set contains the following distinct numbers: 1, 2, 3, 4, 5. We can see that there are no more unique elements in the set, so the cardinal number, which represents the number of elements in a set, is 5.
So, the cardinal number for the set {1,2,3,4,5,4,3,2,1} is 5.