Yes diving is cheating but so is fouling someone who's through on goal and you can do that on the game. If it is introduced, it should be difficult to do though so it's not overused.
The numbers solution's nothing special. 50x(100/25) is 200 so by adding the 75 before dividing by 25, he gets 75/25 more than 200 which is 203. By subtracting 75 he could have got 197 while using similar tricks, it's also possible to get 146, 154, 298 and 302. There are much more advanced tricks ...
How about "Be the advertising boards"?