Play 'number tennis'!

Here is an exercise in using numbers in English that I learned from a book on improving your mental skills. It's pretty hard for native speakers, so it's a real challenge for your learners. I've seen people with a good level in English and an even better level in mathematics struggle with this exercise.
To introduce the game of number tennis I write an incredibly easy equation on the board:23 + X = 100At this point the members of my group normally roll their eyes and look at me in a way that suggests that they think I think they're idiots.
Sometimes I ask them if they did the sum in their own language first and then translated the result. Most people do, and I have to admit that I do the same in French!When you have the correct answer (77, as if you didn't know) give another number between one and ninety-nine and ask them what number they need to make a total of one hundred.
Playing 'tennis' then, involves putting your learners into pairs and getting one of them to 'serve' numbers to the other person who after giving the right (hopefully) answer returns another number.
I referee the game by deciding that a player has taken too long - I shout "out" (how long is too long depends on the level) or if the answer is wrong I just shout no and give the score "15 - love" etc.
You'd be surprised just how hard this is for a learner of English - if the level is low you could start with a total of twenty and then move up by tens. On the other hand, if this exercise doesn't present too much of a challenge, try a more difficult total, say, 250, or if you want to be really sadistic, 573!
Mind you, you'll have to be really good yourself in order to referee (umpire?) the matches...