The theme is standard:

"If there are three people in each of the five boats and four others are in the same boat, then five people have no boat to ride."

It can be understood as: if each ship takes four people, it is just full.

"If there are six people on each of the four boats and the other three people are in one boat, then one boat will be empty in the end."

It can be understood as: if there are three people on each boat, there will be nine people without a boat in the end.

In this way, the number of ships =9÷(4-3)=9.

Number of people =9×4=36 people