If you don't list the equations, the way to solve the problem is as follows:

There are 9 people in each room, 4 more.

Each room 10 people, two rooms are empty, that is, 20 people are missing.

It can be concluded that each room has 9 people, and each room has 10 people. Assuming it is full, there are 24 people short.

24/( 10-9)= 24 gives that there should be 24 rooms.

10 *(24-2)= 220 people or 24*9+4 = 220 people.

If the equation is simpler.

9x + 4 = 10(x - 2)

The solved x is the number of rooms, and then it is easy to calculate the number of people.

I don't know if you understand the above.