let u={5,10.15.20.25.30.35.40

To find the value of "c" given that "u" is a set containing the elements {5, 10, 15, 20, 25, 30, 35, 40}, we need more information or constraints. Without additional information or context, it is not possible to determine the value of "c" within the given set.

If there are specific conditions provided that can help determine the value of "c", please provide them so a more accurate answer can be given.