Calculator
Always use your calculator when calculating.
Explanation
It prevents calculation errors, and you save a lot of time. Do not be ashamed if you use your calculator even for simple tasks. Thereby you develop routine.
With a few keys you can calculate formulas, which is of course very useful. It's great if you can make graphs too. On the graphical calculators you can also calculate intersection points of curves.
In a few years you need no separate calculators anymore, but can you calculate everything on your phone or tablet. You can then also look in mathematics sites, and that makes it even easier.