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I kinda get the concept, but whenever I get a problem like 900-610 or 6,801- 3,923 I get stuck. So can somebody please help me out here, I need to understand this for a test, or quiz.

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Let's use 6,801-3,923.
Ones place. Can't do 1-3, so borrow from the tens to make 1 into 11.
11-3=8.
Tens. 0-2; You already borrowed from this number, and you also can't subtract. Borrow from the hundreds-turn the 0 into a 9. 9-2=7.
Hundreds. 8-9. You borrowed from this number, and can't subtract. Borrow from the thousands, add 10, and subtract 1.. 8 is now 17. 17-9=8.
Thousands. 6-3. Now this you can do, but you borrowed. No need to add a 10 because you can subtract with it. Take away 1. 6 is now a 5. 5-3=2.
You should get from 6,801-3,293 the answer 2,878.

A shortcut I know is that if you try to make 0 subtract-able, it always becomes a 10. But what about if you had to borrow in a number like 100100? You change the first 0 into a 10, but each 0 after that one is changed into a 9. If the pattern breaks, like it does with the 1 in the middle, you just repeat the pattern.

I'll provide links to a couple of sites.
http://www.mathsisfun.com/numbers/subtraction-regrouping.html