Question: I have excel like this.

[![enter image description here](https://i.stack.imgur.com/U4QjK.png)](https://i.stack.imgur.com/U4QjK.png)

I want end result like this

[![enter image description here](https://i.stack.imgur.com/ogl0A.png)](https://i.stack.imgur.com/ogl0A.png)

Means

I want to Put formula in column c to get grade based on what is in column B.

If in B is Check mark, C should show "A"

If in B is Cross mark, C should show "B"

If in B is Question mark, C should show "C"

Answer: 
Control Panel > Region > Additional settings...

Change decimal symbol to comma and it will work as expected. Tried it myself just now and it works like a charm.
As you said yourself, this is a problem with the calculator in Windows 10 that hasn't been fixed yet. What I did to solve this for me was to [install the Windows 7/8 calculator](http://winaero.com/blog/get-calculator-from-windows-8-and-windows-7-in-windows-10/).