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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 keystrokes you can calculate formulas, which is of course very useful. It's great if you can make graphs too. On graphing calculators you can also calculate intersection points of curves.
In a few years, you may not need separate calculators, but you can calculate everything on your phone or tablet. With that, you can already look into math sites now, and that will make life even easier.