If this is a chronic problem, your mistakes may not be careless. There is a type of mistake that will disappear and a type that is related to more fundamental problems of understanding. But careless mistakes are nevertheless a problem; for example, careless mistakes are not permitted of bank tellers, construction workers, airplane pilots, or neurosurgeons. Check all answers for accuracy and reasonability, backtracking line by line; and reserve time on tests for a final check. If you practice being careful as you work homework problems, you can overcome the problem of ``careless'' or ``stupid'' mistakes. But it is interesting that many students would prefer to blame their intelligence or their carelessness before their effort becomes the variable.