There's an even easier way. Start calculator on your iPhone, rotate it to landscape to switch to scientific calculator, type 5, 2, x! and observe result: 8.06

There's an even easier way. Start calculator on your iPhone, rotate it to landscape to switch to scientific calculator, type 5, 2, x! and observe result: 8.06 * 10^67 Yeah, really ingenious, I know... but still.. Just a couple of hours ago, we were playing this math game with my 6-year old daughter and it involved making mental additions and substations to reach a certain number ... After some time she kind of started liquefying in the chair, becoming frustrated and bored with the effort of the mental math... But then I offered her the calculator and she immediately loved it - she could outsource the hard computations to the device and just enjoy the "game" part of the game. At first I went - "oh no, she won't learn how to do arithmetic with her head if she uses the calculator - that's terrible !", but then I noticed that she actually did the easy parts in her head and only used the calculator for 'hard' subtractions. In the end, the fact that we went on and enjoyed the game with the help of the calculator, generated a lot more mental math than if we'd have stopped playing the game. Anyway, not sure what my point is - I guess that sometimes a calculator is even more convenient than clever math tricks - unless your game is the math tricks themselves...