Some researches suggest that Well-designed computer games can facilitate children's learning and motivation on mathematics [11, 17]. To sustain the fun in playing a learning game, according to the concept of flow", students need a series of challenges to match their developing skills. This paper addresses two design questions: 1) How to transform a popular game into an educational game? 2) How to design multi-level challenges to sustain fun for practicing arithmetic in that game? In this paper, the game rules of Bingo are modified to design a competitive computer game, called EduBingo, for practicing arithmetic in a classroom. Based on our previous works and surveys of several digital games, four dimensions of challenges are identified for devising such educational games. A trail test was conducted to preliminarily evaluate the game's effects on learning, and, in particular, the influences of different dimensions of challenges on practice.