Calculator? ad-free upgrade free for Windows Phone for next 20 hours

[Windows Central]

One of the most popular Windows Phone apps is Calculator?, with a ton of features designed to help anyone who needs some serious math problems solved. Earlier this morning its developer Richard Walters sent us a note that the program's in-app purchase feature will be free to use for about the next 20 hours or so.

Full story from the WPCentral blog...