how many elements are in the union of three pairwise disjoint sets if the sets contain 10, 15, and 25 elements? how many ways are there to select a student whose major is in one of the departments of the school of science if there are seven departments in this school with 31, 88, 19, 11, 41, 22, and 17 students in each? (assume that no student can have more than one major.) how many ways are there to select a person who lives on a street with five houses if the number of people in these houses are 5, 3, 2, 7, and 6?