grid.html

<div data-grid="4">
	<div data-span="4"></div> <!-- Select -->

	<div data-span="1"></div>
	<div data-span="1"></div>
	<div data-span="3"></div>
	<div data-span="1"></div> 

	<div data-span="2"></div>
	<div data-span="1"></div>
	<div data-span="1"></div> 
</div>


Write a CSS selector that matches [data-span] elements whose cycling 
sibling sum of the [data-span] attribute value is equal to the parent's
[data-grid]value. In this case its the first, fifth, and last 
[data-span] element.