Professor asked 42 students when they had studied for his class the previous week. Their responses were as follows: 9 had studied on Friday , 18 had studied on Saturday, 30 had studied on Monday, 3 had studied on both Friday and Saturday, 10 had studied on both Saturday and Monday, 6 had studied on both Friday and Monday, 2 had studied on Friday, Saturday and Monday. calculate the number of students who: Did all of their studying on one day​