Let a (101, 202, 303, 404, 505) and b (101, 404). what is a n b?

You should use curly braces instead of parenthesis to indicate set notation. The answer is {101, 404} as these two values are in both set A and set B at the same time. The upside down U symbol means "intersection" which is where the two sets overlap. In terms of a Venn Diagram, you'll have two overlapping circles. In the overlapping region is where 101 and 404 are written. In the region of circle A that is outside of B, you'll have 202, 303, and 505. There is nothing to write in the region of B that is outside of A (since all of B is inside A)

Once again, the answer is {101, 404}