The NumberWiz app was released to the App Store on Friday December 10.
If you want to exercise your mental arithmetic skills then NumberWiz is certainly for you.
It works as follows: You pick six numbers out of two stacks of cards. One of the stacks contains all the numbers from 1 to 10, two of each. The other stack contains the numbers 25, 50, 75 and 100. The stacks are shuffled, so you can't select what numbers you get. When you've selected the numbers, you press a button and then the app immediately selects a three-digit number. Now it's up to you to use the four arithmetic operators to arrive at the three-digit result.
It's quite a challenge and you decide for how long you're going to try and find an answer. However after 45 seconds you have the option to ask the app for an answer. I usually press that button to check if the app suggests my solution.
I've played it a lot myself. It helps to practice and there are tricks you can use, see: http://en.wikipedia.org/wiki/Mental_calculation
There's a help button in the app, which displays a short explanation of the rules.