@SwedishFly
Mental Arithmetic is one of those tests where you need some luck to get a good question pool. I've done it twice at DLR and twice received quite some tricky questions, while other people had super easy ones (talking about fairness of giving everyone the same chance...).
Either way, even with a bad pool you can still solve a lot of questions by knowing square numbers up to 30 and cubic numbers up to 10. It's said that 6 correct answers are enough to pass, which is very doable. Just don't set that as your target, practice a lot, learn some math tricks, and just try to get the best out of it.