Introduction: How to Subtract

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Math problems, specifically subtraction problems are everywhere around us. Everyday we encounter math problems without even knowing that we did.

So, what exactly is subtraction? It is simply the difference between two given numbers.

In this Instructable, I will teach you virtually everything you need to know about subtraction. By the end of this Instructable, you should be able to answer any subtraction problem with only a piece of paper and a writing utensil.


So, lets set the scene.....
Its a cold winter night and you are working on your algebra homework. You turn on your calculator to plug in a simple problem when your TI-83 Plus gives you the low battery warning and shuts down. "NO, NOW WHAT WILL I DO?"

This is where my instructable comes in.

Step 1: Gathering Materials and Stating the Problem

Picture of Gathering Materials and Stating the Problem

So, what exactly is subtraction? It is simply the difference between two given numbers.

In this step you will learn to know that the problem you are answering is subtraction. You will know that you are being given a subtraction problem if:

You see the words "subtract, take away, minus, or from" within the problem. You also have to be firmilliar with the subtraction sign "-"

Example: Subtract 30 from 50. That means take 30 away from 50, or 50 - 30.

So, now that you have stated the problem you must set it up properly. To do this, look at the picture below for help.

Just remember thet the bigger number goes on top and to line up the numbers on top of eachother, so that the "ones" are in the ones place and the "tens" are in the tens place. Also remember that the number being taken ayay is on the bottom of the problem.

Step 2: Solving the Problem

Picture of Solving the Problem

Now that you have your problem set up, you must solve it. This method of solving works with all subtraction problems.

The first step in solving subtraction problems is to "cover up" or ignore the left numbers in the problem so that it appears as if you are now solving a much simpler problem. In picture two, you can see that we covered up the left numbers, now making the poblem read 0 - 0 . Now just solve it.



In your head, think about the problem in an easier way. Personally, I like to think of money. So I think of the problem like this: If I have 0 dollars, and I take away 0 dollars, how many dollars do I have?
See? Thats easy! Zero dollars.


So write 0 under the line in the problem, but be sure to write it under the same column. Look at picture three to see what it should look like.


Now do the same thing, but this time, cover up the part of the question you just answered, and solve the next "mini problem" to the left of it. Look at picture four for help.

Now read the question. You should read it as 5 - 3. If you need to, again, think of it in your head in terms of money. "If I have 5 dollors and I take away 3, now I only have 2 dollars.
Write "2" under the line as you did before but under the 5 and 3. Look at picture five for help.

You have just completed a simple math problem! "Uncover" the entire problem and under the line should be the answer "20"



Step 3: Solving a Problem With Decimals and Borrowing

Picture of Solving a Problem With Decimals and Borrowing
[When you encounter a subtraction problem with decimals, you are usually dealing with money.]

This time, lets solve a problem with decimals.
The example problem for this step will be "20.45 - 16.30"

Now that you know how to solve simple problems, some steps to solving this problem will be more brief.

Start by setting up the problem on your paper. When working with decimals, remember to set up the problem with the decimals on top of each other and lined up, like buttons on a shirt. (pic. 1 & 2)

Cover up everything on the left of the problem, so it reads 5 - 0 (which equals 5) drop the answer under the line. (Pic. 3)

Now work your way to the left by covering up everything so the question reads 4 - 3 (which is 1)
drop the answer under the line. (pic. 4)

You're next step would normally be to go and solve the next set of numbers to the left, but there is a decimal. Simply drop the decimal down under the line. (Pic. 5)

Now here is where things can get a little tricky. This next step is called "borrowing," because you are borrowing a "ten" from the next column. The only time you borrow is when your next "mini problem" won't work. If I didn't borrow, my next mini problem would say 0 - 6 , which would give you a negative number answer.

To borrow, you need to borrow a "1" from the next column to the left. So, cross out the 2, and put a one. Put the one you just took and put it in front of your 0 (pic. 6) The mini problem now reads 10 - 6
(which is 4) Drop the 4 under the line. Go to the next mini problem that says 1 - 1 . This equals zero, so drop it under the line. If you did everything correctly, your answer should read 4.15

To see a video of the subtraction problem above, click on the video.

Step 4: Tips and Other Information

To prove that this method of subtraction works with any problem, I've added a much more difficult subtraction video

Tips:
-Remember that when you are working with decimals, to line up the decimal points like buttons on a shirt (see step 3 and picture three)
-When doing math problems, it's smart to use a pencil with a good eraser to fix mistakes
-Write neatly to avoid number confusion (a sloppy 6 may look like a 0 or vice versa)

Other Information:
-This method takes time and will take practice to master, but it will work with ANY subtraction problem that will give a positive answer and starts with a bigger number.
-Remember that you can only borrow from the column of numbers to the left of you're current "mini problem"
-[This link This Link] will allow you to create your own math questions to solve at your desired difficulty.

And remember, this instructable is meant to teach you how to subtract with the most common and useful method using numbers. Numbers are what you will see more often in the "real world." If you want to learn how to subtract negatives, fractions, or apply subtraction to other mathematical topics, see other instructables.

Thank you for viewing my instructable! I hope that it helped you. If you have any questions about the Instructable, please send me a message and I'll try my best to answer you. Also remember to vote for me in the Burning Questions contest!

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