Consider a group of three students A, B and an other student in between A and B. Choice for a student between A and B is 4. A and B can interchange their places in the group in 2 ways. Now the group of three students (student A, student B and a student in between A and B) and the remaining 3 students can be stand in a row in 4! ways. Hence total number of ways to stand in a row such that A and B are separated with one student in between them = 4 × 2 × 4! Now total number of ways to stand 6 student stand in a row without any restriction = 6! Hence required probability =