iPhones are known to be incredibly intuitive devices, however, even though most people find iPhone's really natural to use and navigate, there are tonnes of useful in-built features that can be easily overlooked.
Quick maths
There's no need to open the calculator app. If you're in a hurry just swipe to the right on your lock screen or drag downwards from the homepage to reveal the search bar. Then type in your calculations and your search feature will reveal the answer.
In addition to this, if you're ever using the actual calculator app and are typing in some large numbers, did you know that if you accidentally fumble and type in a wrong number you can swipe to the left over the numbers to erase the last one you typed.