The document discusses the principal of inclusion and exclusion for calculating the size of the union of two sets. It defines set cardinality as the number of elements in a set. The size of the union of two sets A and B can be calculated as |A| + |B| - |A intersection B|. An example demonstrates using this principal to find the size of the union of two sets where some elements are shared between the sets. The principal is then applied to a word problem about calculating the minimum number of programmers a company needs to hire for different types of jobs.