How to subtract faster than your teacher

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Okay, here is a little secret...

When you subtract a BIG number FROM a SMALL number...

It is the same as subtracting the SMALL number FROM the BIG number and making it NEGATIVE. (And that is much easier to subtract!)

So, now that you know that secret, you can SUBTRACT FASTER THAN YOUR TEACHER!...

What is 274 - 197?

We can do this one column at a time. 200-100 = 100 (No flipping required!) Save the 100 in your head.

70 - 90 is the same as 90-70, which is 20, but it is NEGATIVE. (-20) MINUS 20. Save it upstairs.

And 4 - 7 is the same as 7 - 4... but it's negative! (-3) MINUS 3. Got it?

So that's 100 - 20 - 3 which is 80 - 3 which is 77!


Too easy huh?

If I lost you, go through it again. hehe

UH OH! Another one? ! Let's see here... (genius inquisitive look)...

100 - 800 is the same as 800 - 100, but it is negative. So that is -700. CHECK!

40 - 80 is the same as 80 - 40, but it is negtaive. -40. DOUBLE CHECK!

And 3 - 9 is the same as 9 - 3, but it is negative. -6. WE GOT THIS!

So that is -700 - 40 - 6. UH... That's a little complicated T.D. What ever shall I do!!!?

That means it is time for another SECRET! If all of those numbers were either POSITIVE or NEGATIVE then we handle them better.

The -700 is a NEGATIVE. When we put that -40 in front of it, we are actually subtracting a POSITIVE 40. And it's the same with the 6. We are subtracting a POSITIVE 6.

So we need to change ALL the numbers to either negative, or all of them to positive. I am going to change the 40 and the 6.

But how?

Well, when you SUBTRACT a POSITIVE number it is the same as ADDING a NEGATIVE number. So the MINUS 40 becomes PLUS NEGATIVE 40. Same for the 6.

So now it is -700 + -40 + -6, AH! EASIER! Just add them like you would positive numbers but remember that the answer will be NEGATIVE. -746!

What about this one?

We are ADDING a POSITIVE number, but that -700 is still negative. So we need another trick (Which is just like the last trick.)

When you ADD a POSITIVE...

It is just like SUBTRACTING a NEGATIVE. -700 - -40 - -6. That is like 700-40-6 which is like 660 - 6 BUT IT'S NEGATIVE. -654

What about this? Well we know what we need to do. We need to make them all negative or positive.

-700 + -40 - -6. That is just like 700+40-6 BUT IT IS NEGATIVE. -734

Let's clear this up a little bit...

IF you have a PLUS sign it is like SUBTRACTING A NEGATIVE, which makes it MORE POSITVE.

And when you have a MINUS sign it is like ADDING A NEGATIVE, which makes it MORE NEGATIVE.

One more!

Now I can act like a genius and quickly say ''8,6,4,2 NEGATIVE" (Switching them in my head). But let's go through it.

1,000-9,000 is -8000

200-800 is -600

30-70 is -40

4-6 = -2

Get ready to flip em! PLUS A NEGATIVE.

This is like 8,000+600+40+2 BUT ITS NEGATIVE. -8642!



174,367 - 719,718? Anyone?

Well first I am going to flip it (It seems it's much easier if the first column is normal). And I am going to stick that minus sign off to the side and ignore it for a while.

Don't worry, I'll bring it back later.

700,000 - 100,000 = 600,000

Next group. 10,000-70,000 = -60,000

And: 9,000 - 4,000 = 5,000 (positive 5,000 that is)

700 - 300 = 400

10 - 60 = -50

And 8 - 7 = 1

That's 600,000 - 60,000 + 5,000 + 400 - 50 + 1

WAIT! Don't start flipping! I have another trick (one you wish I told you at first).

This is how you handle zeros. Line up the biggest numbers and get rid of all the zeros they have in common. They have 4 zeros in common. (60,000 only has 4 zeros).

Then subtract (Do this one in your head.) 54

Then add them back. 540,000

Now for the smaller numbers. 400 - 50 = 350 (Do it whichever way is easier to do in your head) + 1 = 351. + 5,000 = 5,351

You could have also done the 0 TRICK on the 400 and the 50

Now add that to the 540,000. 545,351!!!!


Told you I'd add it back

Watch the video: Fast Subtraction trick for kids. Easy mental math strategies

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