Doubling and Halving etc, is a useful mental strategy if you are horrible at keeping numbers in your buzzing brain. If you don't like Einstien formulas to find out such multiplication problems such as 66x6, then this is for you! All you do is divide one by 6 and times the other by 6 etc. and wolah! It instantly becomes 11x 36 or 396 x 1! Splendid! You don't have to scribble on ten thousand pieces of paper to work it out.
It can be a pain sometimes espically if it goes off into 3 digits but I rate this method 80 out of