When you are preparing for your numerical reasoning test, you should think carefully about where and when you practice. The best environment for practising, it goes without saying, is one which is quiet, comfortable and familiar.
In addition you should consider the following:
Interruptions: put a sign on your door to let your housemates know that you are not to be disturbed.
Same desk: if your real test is online, and therefore can be taken wherever you like, try to make it the same place as where you have practised (e.g. your room or study). This repetition and familiarity will help replicate your performance.
Noise or disturbances: pay attention to your surroundings; is there an annoying draft, humming, or do you have a wobbly desk?