The restaurant

Twenty persons are eating in a restaurant. They have to pay 200$ in total. Men pay 20$, women 7$ and children 3$. How many men, women and children are in the restaurant?

The swimming pool

A man owns the swimming pool that you can see in the picture (a square with a column near each corner). How can he double the area of the pool without changing its shape, without moving the columns and keeping the columns outside the pool?