How to Subtract Faster Than Your Teacher by Tutorial Doctor
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1 Okay, here is a little secret...

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

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

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

5 What is 274 - 197?

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

7 70 - 90 is the same as 90-70, which is 20, but it is NEGATIVE. (-20) MINUS 20. Save it upstairs.

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

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

10

SEVENTY-SEVEN IS THE ANSWER!

11

Too easy huh?

12

If I lost you, go through it again. hehe

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

14 100 - 800 is the same as 800 - 100, but it is negative. So that is -700. CHECK!

15 40 - 80 is the same as 80 - 40, but it is negtaive. -40. DOUBLE CHECK!

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

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

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

19

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.

20

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.

21

But how?

22 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.

23

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!

24 25 We are ADDING a POSITIVE number, but that -700 is still negative. So we need another trick (Which is just like the last trick.)

26

When you ADD a POSITIVE...

27 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

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

29 -700 + -40 - -6. That is just like 700+40-6 BUT IT IS NEGATIVE. -734

30

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

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

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

33 One more!

34 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.

35 1,000-9,000 is -8000

36 200-800 is -600

37 30-70 is -40

38 4-6 = -2

39 Get ready to flip em! PLUS A NEGATIVE.

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

41 TA DA!

42

THE MASTER CHALLENGE!!!!

43 174,367 - 719,718? Anyone?

44 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.

45

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

46 700,000 - 100,000 = 600,000

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

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

49 700 - 300 = 400

50 10 - 60 = -50

51 And 8 - 7 = 1

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

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

54 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).

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

56 Then add them back. 540,000

57 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

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

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

60 NEGATIVE!!

61

Told you I'd add it back

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