Final answer:
There are 15 unique ways to arrange 4 out of the 6 swimmers for the relay race.
Step-by-step explanation:
To find the number of unique ways to arrange 4 out of 6 swimmers, we can use the combination formula.
The formula for combinations is given by nCr = n! / (r!(n-r)!), where n is the total number of swimmers and r is the number of swimmers we want to choose.
In this case, we have 6 swimmers and we want to choose 4 of them, so the calculation would be -
6C4 = 6! / (4!(6-4)!) = 6! / (4!2!)
= (6 x 5 x 4 x 3) / (4 x 3 x 2 x 1)
= 15.
Therefore, there are 15 unique ways to arrange 4 out of the 6 swimmers for the relay race.