First of all, you need to know all your units of measurement and how to convert them e.g 1mg is equal to 1000mcg. You need to know all that. It would be wise to get or make a flash card and study it.
Read the question carefully
You need to know what the question is asking. All the units in the question might be in ml/minute and then the answer the question is asking for is in ml/hr. So you need to be careful about this.
What is the question asking you to round off the answer to?
Now you need to know how to round off to the nearest tenth or nearest whole number or what ever the question is asking, why? Because if the question says round off to the nearest whole number and you leave the answer as 9.25, you will get it wrong, the answer should be 9.
KNow all the calculations
There is no one particular calculation that you have to know, you need to know all of them. The best way to do this is to buy a nursing mathematics or calculation book and do practice questions from it. The one from Kee i think is a good one to purchase.
I hope this short article has been of help to you and good luck.