Before Angry Zombie Ninja Cats on Xbox 360, I published 2D Math Panic in Fall 2011. After reading an XNA book, I built the simple math game in 1 evening after work, followed by additional tweaks during playtest and review. It also helped me assemble the XNA Basic Starter Kit and create the XBLIG Sales Data Analyzer in 2012.
Download here: http://tinyurl.com/2dmathpanic