Problem with odd/even child elements in nth-child

Question

I have a web site like this:

Answer 1

div > :nth-child(3)     the third child of a div element
div > :nth-child(even)  every even child of a div element
div > :nth-child(odd)   every odd child of a div element
div > :nth-child(3n)    every third child of a div element