Just like the solution to the magic square riddle, this puzzle is solved by using the next dimension. I don't have any magnetix with me so I combed youtube to see if anyone had this. I know some of my students posted pictures and/or videos to facebook, but I never saw them.
You have to spin the icosahedron so that it can "show" you the answer.
I have no idea who this guy is and he never catches the image made in the center. Bummer!
You have to look at it straight on and you will see [X X X] plain as day. All my students could see it. And, of course, you need to know that XXX is Roman numeral for 30, hence the 8th grade education.
It is actually pretty cool, and most of the students loved creating it and others and spinning them to see whatever. Nobody showed me this optical ilusion, I just discovered it one day from playing with the magnetix.
May the words of my mouth and the meditation of my heart be pleasing in your sight, O LORD, my Rock and my Redeemer. Ps 19:14 (NIV)