Fingu is a game and a game platform aimed at helping children 4 to 8 years old develop competence and fluency with basic addition combinations. We present results from an eight week intervention, where children, 5-, 6-, and 7 years old, have been allowed to play the game as part of their ordinary pre-school and school activities. Results show significant positive differences between pre- and post-tests in four arithmetic measures with modest to large effect sizes. In contrast most differences between post-tests and delayed-tests, when the children did not play the game, are non-significant with none or low effect sizes. Design principles and results are discussed with special emphasis on children who “struggle to learn math”.