#6**+9 **

For seven:

Take the last digit, double it, and subtract it from the rest of the number. If the answer is divisible by 7 (including 0), then the number is also divisible by 7:

861:86-1x2=84

84/7=12

thejamesmachine
May 27, 2015

#1**+8 **

Try them all out

1&92, 2&46, 4&23,

2 - if it is even

3 - if they add up to something divisible by three

4 - ends in 0 or something divisible by 4

5 - ends in 0 or 5

6 - is divisible by three and two

9 - is divisible by three and try it

10 - ends in 0

11 - a double number (ex 55)

Guest May 27, 2015

#2**+9 **

That 11 one only works under 100.

Try this:

1. Add the digits of odd positions from left to right of the number.

2. Do the same for the even positions of the number.

3.Find the differences between them.

4. If the difference is 0 or a multiple of eleven, then the origional number is divisible by 11:

1320 1320

1+2=3 3+0=3

3-3=0

1320 is divisible by 11!

thejamesmachine
May 27, 2015

#3**+4 **

To work out if it is divisible by twelve, see if it is divisible by 3 & 4.

thejamesmachine
May 27, 2015

#5**+9 **

For eight, the last three digits are divisible by eight, then the whole number is.

thejamesmachine
May 27, 2015

#6**+9 **

Best Answer

For seven:

Take the last digit, double it, and subtract it from the rest of the number. If the answer is divisible by 7 (including 0), then the number is also divisible by 7:

861:86-1x2=84

84/7=12

thejamesmachine
May 27, 2015

#8**+3 **

Thanks Thejamesmachine,

Thats a neat summary of factor shortcuts. I had not seen the 7 one before!

Melody
May 27, 2015

#9**+5 **

Alan has given a proof for the 7* one here

For seven:

Take the last digit, double it, and subtract it from the rest of the number. If the answer is divisible by 7 (including 0), then the number is also divisible by 7:

861:86-1x2=84

84/7=12

http://web2.0calc.com/questions/prime-factorization-of-624

Thanks Alan and thejamesmachine

I'll add this to out sticky notes Reference material under "Determining factors of a large number"

Melody
May 31, 2015