Anyone know where to practice logic sequences (example inside)

Here's an example:

http://imgur.com/cNlSJ

I can see the pattern of the inside shape, but not the outside shape. Thankfully for this one that was enough (anyone know how to find the outside one).

Do you guys know where I can get examples and explanations and practice for these problems online? SHL doesnt give out explanations so if you cant solve one forget it.

Thanks!