I suggest preparing for GMAT (Problem solving and Data sufficiency) (in English or in German). I will help you to find a creative approach to solving your problems. I finished one of the best high school with a major in physics and mathematics. I have also competed in various math competitions and won many of them. Now I teach math myself, and my students are getting success in taking exams and improving their grades. I have been working as a private math teacher for 6 years.