You can do a lot more than just simple addition and multiplication in Spotlight. You can use parenthesis to set the order of operations. You can use functions and symbols to do trigonometry and logarithms. You can use powers and roots, and even scientific notation. You can even do factorials, percentages and trig functions using degrees instead of radians.
If you searching your Mac for a file doesn't return the results you expect, then there are several common culprits. You may simply not notice that the scope of the search is limited to the current folder instead of your entire drive. Or, you may have your Spotlight settings set to not look for some files. If there is a more serious problem, you can try re-indexing your Mac to fix broken search results.
When using a computer, you usually don't have a multiplication or division symbol on your keyboard. To do basic mathematical functions, you use the asterisk and slash keys instead. Some apps, like Numbers, will even convert these to multiply and divide symbols. At other times you will still see those characters in your math equations.
Users who are new to Mac often don't know about Spotlight, or think it is just a way to search for files by name. You can use spotlight to search for files by name and content and preview the contents before you open the file. You can also go to the file instead of opening it. Spotlight will also find other items like dictionary definitions, email messages, images and more. You can use Spotlight to do mathematical calculations, unit and currency conversions. One thing that many people use Spotlight for is a quick and easy app launcher.
El Capitan adds several new features to Spotlight searches, including the ability to search for weather, sports information and stock prices. You can also search your own files using natural language such as 'documents I modified yesterday.' You can customize what shows up in Spotlight searches in System Preferences.