Subtracting From Two-Digit Numbers

Two-digit numbers? It will be as easy as pie now that you can do subtraction through twenty (20) and you know about regrouping in subtraction. Most of our examples will have you subtracting single-digit numbers from double-digit numbers, so the subtraction will be a bit easier. It will help to set up the problems in the up and down (vertical) format so that the columns match up.

Column by Column

Remember that you always work from right to left. The ones column is on the far right side and then tens, hundreds, and thousands. Since we're working with two-digit numbers, you only have to worry about ones and tens.

Examples (with no borrowing):
12 - 1 = 11
28 - 6 = 22
34 - 2 = 32
68 - 5 = 63

Do you see how, since there is no borrowing, you just bring the tens column down with no subtraction? You're doing a subtraction problem, but you are subtracting zero, which means the original number (minuend) will be the same. Subtracting these combinations of numbers are as easy as single digit subtraction. "5 - 3 = 2" will help you in any problem from "85 - 3" to "1,584,687,265 - 3." Patiently moving from right to left makes it very easy.

Now a Little Borrowing

Not all of the subtraction problems will be simple. You will need to borrow/regroup about half of the time. It's not harder. It's just different. As a reminder, the concept of regrouping in subtraction has you "borrowing" a "1" from the column to the left. Since we're only working with two-digit numbers, you will borrow from the tens column.

54 - 5 = 49 (after you borrowed you made 14-5 in the ones column)
62 - 7 = 55 (after you borrowed you made 12-7 in the ones column)
21 - 4 = 17 (after you borrowed you made 11-4 in the ones column)

Remember that you can check your work by flipping the symbols and using addition.

54 - 5 = 49 (check to make sure that 49 + 5 = 54)
62 - 7 = 55 (check to make sure that 55 + 7 = 62)
21 - 4 = 17 (check to make sure that 17 + 4 = 21)

Know the Basics

When you break down all of these steps, the most complicated subtraction problem happens when you subtract a single-digit number from a double-digit number. You may do this step over and over, but it's just a simple subtraction problem done a bunch of times. By the time you can work with numbers up to twenty, there's not much more to learn about subtracting whole numbers.

Done one time: 14 - 5 = 9
Done twice: 154 - 55 = 99
Done three times: 1,554 - 555 = 999

By the last example, it's the same subtraction problem three times in a row. As you work out more problems you will start to see the patterns and shortcuts. Getting better at math is all about practice, practice, practice.


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Numbernut: THree-Digit Subtraction
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