How do you know if you need to ungroup a ten for ones when subtracting

Question

How do you know if you need to ungroup a ten for ones when subtracting?  My son came home from school and did all of his math homework except for the stretch your thinking part...I don't know how to answer that question.  

Answer 1

Subtraction is the inverse of addition.  Suppose we are adding numbers in columns for example:

 

   64

+ 96

 

 

4+6=10 , but in this case we put a 0 in the one's place and carry the 1 over in the tens place.  Subtraction works the same way, but the only difference is that you work backwards by borrowing.  Here is an example.

 

  64

- 36

 

Instead of doing 4-6 in the ones place, we perform  14-6,  and then replace the 6 in the tens place with number 5.  This technique is called borrowing.  So your solution to this subtraction example is 28.

 

I hope this helps answer your question.