Hey, i love this game. simple, addictive, and pleasing to math brain.
May I make a request / suggestion? Add a custom mode. Going in, you can choose the amount of total shells, and the ratio of lives to blanks, but not the order they go in. You can also choose if items are part of that round, and potentially what items for true customization, but the randomness is part of the fun.
The reason I suggest this, is this can become a great way to test / teach probability, and by setting up the same setting and running it repeatedly, you can actually experiment with it.