Nice math practicing machine. However, do have some gripes, of course :) Please implement ENTER instead of having to click on 'Check' with the mouse. I love to stick to keyboard when the mouse is not really needed for anything (or vice versa).
Also, the levels of difficulty could use some love. I don't think 30 * ??? = 1560 is easy enough for a child, as I also don't think 344 + ??? = 716 is hard enough for a genius ;)