That damn equation keeps popping all over the place in Equestria. It was on the board when young Twilight hatched Spike, it's in the schoolhouse, and now it popped up in Twilight's Failure/Success song.
I figured the brightest minds around would be working on this problem. I put the five minds I thought would work best on this. The pony with the sweater is the profesor pony that says "She is correct!" during Twilight's song. If you're looking for the fifth pony you need only look to the reflection in the window. She was a last minute addition so she didn't get a formal spot with the others.
I had fun doing the titles for the books!
MLP:FiM belongs to LF and Hasbro