I would recommend looking on YouTube at ‘Vedic’ math tricks for mental maths. There’s a series on there with a turtle in the preview image which helped me a lot in preparing for aptitude tests etc. Those videos will help with learning techniques to do more complicated maths quickly but the basics are only learnt from lots of practice. If you spend an hour or two a day for a week doing basic addition and multiplication then you should have the basics down and the rest will get a lot easier, especially when under pressure