how does this work??

doesn't work for me,
i picked 1212 then scrambled to 2121. 21-21=0

but then again i like to screw with the system. i remember these math puzzels as a kid.

 

it was confusing instructions, but they meant for you to do 2121-1212=909

 

miss read the sub question nevermind.
it has to do with adding the digits and subtracting over 9.
use primes
1357
3751
2394
circle 9, add 2,3,4=9 the missing number
circle 4, add 9,2,3=14-18=4
circle 3, add 9,4,2=15-18=3
circle 2, add 9,3,4=16-18=2

 

another
5368
6538
1215
C,1 add 215 = 8-9= 1
C,2 add 115 = 7-9= 2
C,5 add 121 = 4-9= 5

 

it's a 9 thing, less than 9 subtract by 9 becouse its aone digit number, equal to 9 then the answer is 9. greater than 9 subtract by 18 becouse it's a two digit number.

 

I used:

1126
6112
4986
circle 8, add 4,9,6=19 ....?

 

soory for numbers greater than 18 subtract 27 (9*3) and so on. based on 9's
9*1=9
9*2=18
9*3=27

see the pattern?

 

Cool. I knew you said it was based on 9's, I just didn't get it with my example. I see the pattern now of course. Thanks....