[填空题]
Ten students are to be arranged in a new chemistry lab. The chemistry lab is7cqz8 oll l)gz57dfe / sd5dufn3jcw x5 ay:7-rz4pr ; qet out in two rows of five desks3pd-xrf:yn4r5j ; 5wduzqa 7c as shown in the following diagram.
1. Find the number of ways the ten students may be arranged in the lab.
Two of the students, Hugo and Leo, were noticed to talk to each other during previous lab sessions.
2. Find the number of ways the students may be arranged if Hugo and Leo must sit so that one is directly behind the other. For example, Dest 1 and Desk 6 .
3. Find the number of ways the students may be arranged if Hugo and Leo must not sit next to each other in the same row.
