Ronaldo
Honestly, there's no real trick for that.. you just have to enhance your mental math. Hopefully, you've been prudent enough to improve mental speed calculations months prior. But it's still alright if you can't get the hang of it, just keep on practising additions, multiplication, division and subtraction. But with a time constraint, so just try and be faster with your calculations basically. As that is what most of numerical and math tests you on.
