Final answer:
Out of 18 students, 12 eat in the cafeteria, which in simplest form is 2/3 of the students.
Step-by-step explanation:
If there are 18 students in Jacob's homeroom and 6 students bring their lunch to school, then the number of students who eat lunch in the cafeteria is the total number of students minus those who bring their lunch, which equals 18 - 6 = 12. To express this as a fraction of the total number of students eating in the cafeteria, we divide the number of students eating in the cafeteria by the total number of students. Therefore, the fraction of students eating in the cafeteria is 12/18. Simplified, this fraction becomes 2/3, since both the numerator and denominator are divisible by 6.