A few weeks ago, the Mathematical Ninja explained how to divide by 9 on the fly. This week, it's multiplying that's on his mind - a more common thing to have to do.
The multiplying method is easier, too. It's just a case of adding and taking away - and writing a few numbers down. In this version, you work from left to right, which is quite the opposite of what you're used to.
Here's an example: $6134 \times 9 = 55,204$. How you do it:
This is quite a tricky one to do in your head without practice, but doing it on paper can save you a lot of carrying - and once you've got the hang of it, you only need one line of working!