In addition to Interval Arithmetic (http://www.windowsphone.com/en-us/store/app/interval-arithmetic/02f62df9-0436-4ae0-931c-1848b04d68e1), I have a few more apps in the Store.
Playdough, compatible with Windows Phone 8 and 8.1 (http://www.windowsphone.com/en-us/store/app/playdough/ab3a9bd3-c963-4882-8b67-e9d20fd4a522). Playdough is for folks who have a mortgage and a little extra money each month. Once you enter the necessary facts and figures, the app will help you explore whether you're better off paying down your mortgage as soon as you can, or whether you should keep that debt and invest all you can. The data you enter into the app remains on your phone at all times. That makes it as private as your phone is. Playdough is provided for education and informational purposes only.
Numeroids, compatible with Windows Phone 8.1 (http://www.windowsphone.com/en-us/store/app/numeroids/aa5858b8-c266-4f1f-83e7-375ff0e24fba). Numeroids will sharpen your mind and exercise your ability to perform mental arithmetic. Don't allow your mental faculties to get flabby. Stop relying on an electronic calculator to perform simple arithmetic. Install Numeroids and play with it whenever you have a spare moment and see how your thinking power improves!
NumeroidSL, compatible with Windows Phone 8 and 8.1 (http://www.windowsphone.com/en-us/store/app/numeroidssl/6b9f8036-6ab6-4ba9-ad3f-bdf403a93466). Same as above.