Practice a lot or try to learn Swift Maths. You will know how to do calculations quickly.

watch speed math series on youtube by sreedhar's cce.
https://www.youtube.com/watch?v=z5bLbt78254&list=PLM6jca0G0xvvOudToBYNyEugihtj3iDNw
The method used in the series is very effective. Plus you want a lot of practice