I would like to improve my mental arithmetic. While okay in some areas, I am bad in others. I graduated in (pure) mathematics for my undergrad, while never having learnt my multiplication tables fully.<p>I have had the Trachtenberg book lying around for years, but I am not sure whether it is worth the time investment. I would also prefer having a computer programme which went through it.<p>I would like to know what HN's experience is with improving mental arithmetic.<p>1. Have you always been naturally good? If so, what do you think about existing systems?<p>2. Were you bad but became good with a certain system?<p>3. Have you tried multiple systems? How do they compare?<p>4. Did you take classes or do it independently?<p>5. If system, what age did you learn?