65. A workshop has a production line consisting of 5 machines. Only when all 5 machines are operating can this production line work. There are a total of 8 workers working on this production line. In each workday, only 5 of these workers show up. To ensure production, the 8 workers need to be trained. One worker learning the operation method of one machine is considered one training session. Therefore, the minimum number of training sessions required to ensure that the production line can always work when any 5 workers are present is $\qquad$.
answer: 20