the difference –Discovery Math

Ahhhhh, I get it now.  Thank you both so much!  Jo, when I first read through your answer I was mega confused.  After I read Suzukimom’s explanation which was so simple and was easy to grasp,  I went back and re-read yours and the lightbulb came on.   I think I got hung up before because I wasn’t thinking about how we actually would add the 20 and 7 together when doing math the “borrow and carry” way. 

Thanks for your patience ladies.

I know you had your aha moment…. just one other quick way to look at why I was adding the -3 instead of subtracting….

When subtracting (in columns), you figure out the ones, you figure out the 10’s, etc… and add the results of each column(* the place value of the column)…  

so with something like 34-1 you subtract the 1’s, (4-1) and get 3… and you subtract the 10’s (3-0=3) and add the 10’s * 10… 30 plus the ones times 1 (3) and get 33.  Of course with the original problem of 34-7 you do the borrowing, and it comes to 2 in the 10’s column and a 7 in the 1’s column so you do 20+7=27.

with me, my 10’s column was 30, and my 1’s column was -3 and when I added them I’d get 27.

I can think of a few ways for this.