In your dining room, there are P people, C armless chairs, and one table with four legs. There are 10 arms in the room and 18 legs. How many more chairs do you need to get so that everyone can sit down?
Since the chairs and the table have no arms, the 10 arms belong to 5 people (assuming that each person has 2 arms).
Therefore, the 5 persons also have 10 legs (again assuming that each person has 2 legs).
Since there is a total of 18 legs, 8 legs belong to the table and the chairs.
Since 4 of the legs belong to the one table, the remaining 4 legs belong to 1 chair.
We have 5 persons and 1 chair; it's time to get 4 more chairs.