I can multiply any three-digit number by 1667 in my head, usually in less than 10 seconds.

Also, to make it even more impressive, I come up with the digits of the result in proper order (not in reverse!)

How can I do this?

mohan,
I think we all see your method for multiplying by 10002 (push the original digits up 4 places (x10000) and tack on 2x the number)and that it is straight-forward. What I, for one, can't figure is how dividing a 7 digit number, that you've only just derived (and therefore hanging around nervously in your short term memory) by 6 is easy.
On paper it is easy but in your head?
e.g. 6741348/6 =
"okay, six into six goes once .."1"
six into seven goes once with 1 remainder.."1"
six into fourteen goes twice with 2 remainder.."2"
six into twenty-one goes thrice with 3 remainder.."3"
six into thirty-three goes five times with 3 remainder.."5"
six into thirty-four goes five times with 4 remainder.."5"
six into forty-eight goes eight times exactly.."8""

Possibly, but I don't think my mind could cope.
Also the original question is how the poster, nellie, does it. As s/he claims not to fully understand how they do it, it seems unlikely that they go through this memorable long-division procedure. I feel it's along the lines of what Charlie and Gamer have considered and has more to do with 'patching' numbers together than doing a conventional approach
Just my 2p

Posted by Lee on 2003-10-13 12:26:47