Casting Out Nines

snowbird

I learned how to do this in 4th grade, but got rusty over the years.
A refresher course helped me to see that it really works!
Also, if you want to know if a number with 2 or more digits is divisible by 3, add the digits - which sum will be smaller - to see if the sum is divisible by 3.
Sylvia
snowbird

I always forget the link.
http://www.themathlab.com/natural/mental%20math%20tricks/casting%20nines/casting.htm
Sylvia
snowbird

Nobody interested?
Sylvia
dinah

I thought maybe it was similar to casting out demunz.......
snowbird

LOL at Dinah.
You're a hoot, girl.
Sylvia
John Doe

Interesting. I've never seen that.
Sad emo

I got confused after the adding up bit!
But it was fascinating - seems that Asimov's genius may have been grossly underestimated by some.
I might go back to try get to the free gift page later when my brain works better!
snowbird

Back in the day (early 60's), when I was in elementary school, we learned all sorts of stuff.
The teachers were given free rein, and they used it to their and our advantage.
Sylvia
snowbird

Emo, I used to be able to do it really fast!
I would always get the prize from Teacher - a nice, thick ginger cake.
Sylvia
snowbird

There are other tricks I remember vaguely.
E.g., if you are multiplying a number by 25, 250, 2500, etc., simply divide multiplicand by 1/4.
Same thing for 50, 500, 5000, except you divide by 1/2.
Sylvia