The best approach to take here is to practice your mental maths. Without trying to be patronising, finding websites or apps which give rapid-fire complicated mental maths questions, such as multiplication, division, percentage calculations, etc. can be a goldmine for passive skill development. Do these for 5 minutes at a time whilst you’re waiting for the bus, in the car, or even at the dinner table and what you will observe is a gradual development in your mental maths. The benefit is this saves you vital time (you only get around 40 seconds per question ) especially as you won’t have to use what can the on-screen calculator, which can be problematic for many students.
However, sometimes it can be beneficial to take a step back. Despite seeming counterintuitive, ‘guesstimation’ is a vital skill during exam conditions. For single-step questions, by eyeballing the answer options present and appreciating differences between them, e.g. orders of magnitude between options, can lead to you eliminating two or three answer options of the five even before any calculations if the options do not seem realistic. As a result, even if you were to guess now, your chances of getting the right answer could have improved to 50%! As a side-note, please do guess if you don’t know the answer, don’t leave a question blank – there is no negative marking so you don’t risk losing marks if you guess. Compared to the 100% guarantee of losing a mark, you have at least a 20% chance of getting it right by sheer chance.