In a certain city houses had to be built in a particular way.
There had to be two rooms on the ground floor and all other rooms had to be built on top of these.
Families were allowed to build just one room for each person living in the house.
So a house for two people would look like this:
but a house for three people could look like one of these:
There are some families of seven people living in the town.
In how many different ways can they build their houses?