You don't need an app to do basic math in Windows 10. If you've enabled Cortana, you can perform those calculations by tapping the Windows key and then typing the formula in the search box.
Make your formula as simple or as complex as you want, using standard operators: + and - for addition and subtraction, * and / for multiplication and division, % for percentages, ^ for exponentiation. Cortana automatically groups internal calculations; use parentheses if you want to control grouping.
Cortana displays the result of your calculation at the top of the search results list.
Want to do more calculations using the result of that formula? Press Enter to open the full calculator shown here.
Note that the calculator vanishes as soon as you click away, and you can't copy the formula or its results.