Why does my mother force me to do mathletics and get 90 or above?
It's good that she is making you get good grades, and that isn't something to complain about. Most mothers these days don't care and some will not even make their kids go to school (which is illegal, not to send your children to school). But for mathletics, do what you want, not what she wants. If you are interested in taking part in it then go ahead. Other than that, it's not your mothers job to tell you what kind of activities or groups you should be interested in and joining.