Lesson 5 Subtracting Whole Numbers 
Important Note from the Author It is likely that you have already mastered the procedures for subtracting whole numbers, and have applied them successfully for a long time. However, this review of addition might not be a total waste of your time, because it points out some definitions and underlying principles that need to be more clearly recognized in today's workplaces and learning environments. 
Subtraction is the reverse of addition:
Topic 1
Introduction to Subtracting Whole Numbers
Definitions
The minus sign (–) indicates the subtraction operation. 
Subtraction problems are sometimes written in a horizontal form such as:
This form is also known as a number sentence. It is read as, "Five minus three equals two."

Notes

Important When you are subtracting whole numbers, the subtrahend must be less than the minuend. Or in other words, you should not try to subtract a whole number from a smaller whole number.
There are no negative values in the whole number system. 
When subtracting pairs of whole numbers that have more than one digit, you must subtract digits that have the same place values—subtract the ones digit in the subtrahend from the ones digit in the minuend, the tens digit from the tens digit, the hundreds digit from the hundreds digit, and so on. So when you are setting up subtraction operations in the vertical form, always begin by aligning the place values.
For example:  Then subtract each of the columns from right to left. Write the difference digits under their corresponding place columns. 
Topic 2
Subtracting Whole Numbers Larger than 9
Procedure When subtracting a pair of whole numbers larger than 9:

Example
Topic 3
"Borrowing" for Subtraction
"Borrowing" is necessary whenever you face a situation where you are trying to subtract a larger number from a smaller one.
27 – 4  This example does not require borrowing, 
72 – 4  But this example requires "borrowing," because 4 cannot be subtracted from 2 in the wholenumber system. 
The idea behind borrowing is to add 10 to the top value. In our example here, you can add 10 to the 2. Now you can subtract 4 from 12. That works. But it is not a good idea to simply pull the number 10 out of thin air. That's illegal. The 10 has to come from somewhere, and that "somewhere" is the nexthigher place value. In this example, subtracting 1 from the 7 makes that extra 10 available for changing 2 to 12. 
Borrowing is often necessary for completing subtraction problems. 
Procedure When you are working a subtraction problem, and you find you are trying to subtract a larger value from a smaller one, add 10 to the top value by subtracting 1 from the nexthigher place value. 
Use these interactive examples and exercises to strengthen your understanding and build your skills:
If would like to see more examples or work on a wider range of exercises, you can go to a more detailed tutorial . 