With the undos, I don't completely get what the rounds are doing. I'm not sure if I used an undo - I clicked on a number (was going to divide another number into it), but then I realized that I needed to do something to the 2nd number first, so I clicked on the first number again to un-select it. Was that an undo?
I like the extra info provided as well, about how many ways to get that total there were, and how many totals those numbers could make.