Jan 31, 2012

Subtracting Apples from Bob

HT here.
This week, we will focus on subtraction. Which is as easy as taking apples from Bob.
Subtraction is the exact opposite of addition, so the methods you've learned for doing addition can be used for subtraction, with slight tweaking.

Take 6-4 for an example. We will use the l's method, thus;

As you can see, it's very similar to the l's addition method that you learned.
All you need is a good grasp of addition, and subtraction will be as easy as stealing cookie jars from the cookie. Oh, I mean, cookies from the cookie jar.
Once you have single-digit subtraction down-pat, 2-digit, and even larger subtraction problems will be a piece of cake. Work from right-to-left, subtracting the 1's first, then the 10's, and so on.
Take 17-11 for example. Subtract 7-1 to get 6, and subtract 10-10 to get 0. So your answer will be 06, or since the zero is only a placeholder, and not needed at the beginning of a number; 6.
Sometimes, you will get a scenario like this;
32-27=... dividing it into 1's and 10's, you get 2-7 and 30-20. Starting with 2-7, we run into a problem. 7 is greater than 2! What do we do? Simple, cross off the 3 in 32, and replace it with a 2, like this 20-20. now add that 10 to the 2, like this 12-7. Now it's easy to find the answer. 12-7=5, and 20-20 = 0, so your answer is 5
If the number you are subtracting with is greater than the number you are subtracting from, the easiest way to figure out the answer is to reverse the subtraction and then add a "-" sign to the answer.
Thus. 45-47=, 47 is greater than 45, so, switch it around, and subtract 47-45 to get 2. now, simply add a "-"sign to the answer, -2

Alright, here're some test questions to get you started. And remember, as before, keep your eyes open for subtraction problems.


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