How to master doing calculations by hand, some tips and tricks?

So in my semester we have statistics as a subject, in it there is a chapter about probability distributions. I struggle at long calculations decimal and no way can I completely calculate normal distribution [1/std*sqrt(2pie)]e[(x-u)2 * 1/2std2] by hand down to decimals. But I have no choice other than doing it by hand as calculators are not allowed in exam. How do you guys did it in your exams? Please give some tips and tricks to this rookie.