ive noticed that I've been making too many joke posts recently so i need to even out the quota
firstly, you make silly mistakes because you suck at tests. It is important for you to realise this. The problem is either a) you are far too elite for the test so just take shortcuts (e.g. archman, ngai) or b) you suck. Either problem, it is important for you to come to this realisation. Admitting you have a problem is half the solution, or something. You've probably taken offence to the suggestion that you suck. But you do. Face it. You don't get 100% in tests because you don't deserve 100% in tests. But it's OK, most people suck.
Now you might be wondering 'Rorix, how do I suck less?'. I don't know because I also suck. But I have some suggestions. Firstly, doing more papers can't hurt. Make sure you do them with worked solutions and go back over answers you got wrong to see if you keep making the same mistakes. Secondly, don't perform 12 operations on the equation in your head without writing something down. Don't worry, no-one will think you have a smaller manhood for not doing questions by inspection. Now, quickly check the numbers from line to line if you did something complex, it's quicker than checking at the end of the test. Note this doesn't involve redoing the whole thing, but if you get a fraction out of 6 - 3 you might just have a problem.