We need to find the number of full tables, to do this you must keep in mind three important things.
1. there are 37 people
2. the tables are filled with 2 people
3. One table is not full
If the only way to fill a table is with two people at it, it means that we have to arrange the 37 people at the tables and see how many are filled in total.
i.e. divide the 37 people in 2 which is the capacity of the tables.
since you can't have 18.5 tables full we approximate 18, this is accurate since it also tells us that there is one table that is not full.
In conclusion, the cafeteria has 18 full tables