One of the most popular Windows Phone apps is Calculator², 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.

The deal allows users to unlock the app so that no ads show up while using the calculator, while also adding a background task for automatically updating the exchange rates.

We mentioned that the app was pretty popular, but how popular is it? Well, Walters told us this morning that the previous version of Calculator² for Windows Phone passed the one million downloads milestone last week. The Windows 8.1 version reached that same mark back in October.

However, that older Windows Phone version of the app will soon be removed from the store in favor of the current Calculator² app, which used to be called Calculator³. (Yes, we know it's a little confusing). The reason is that the newer version has been updated to become a universal app so that unlocking the in-app purchase will also work with its Windows 8.1 counterpart.

You can download the free Calculator² app right now from the Windows Phone Store and you can also get the Windows 8.1 version from the Windows Store.

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