A university professor asked all his 42 students in class when they had studied for his class the previous weekend. their responses were as follows: 9 had studied on friday; 18 had studied on saturday; 30 had studied on sunday; 3 had studied on both friday and saturday; 10 had studied on friday and sunday; 16 had studied on sunday only; and 2 had studied on friday, saturday, and sunday. how many students studied on sunday but not on saturday? how many of the students did all their studying on two days? how many students did not study at all for class last weekend?