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Adding & subtracting rational numbers: 79% - 79.1 - 58 1/10

Sal evaluates 79% - 79.1 - 58 1/10. Created by Sal Khan.

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  • winston baby style avatar for user Germaine Johnson
    I still don't get how are you supposed to covert it into a fraction like you did in the video.
    (23 votes)
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  • starky sapling style avatar for user abigailchoi77
    How do you make a fraction a decimal?
    (8 votes)
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  • starky ultimate style avatar for user Yug Shah
    At , Sal factors out a negative sign. Can someone explain to me what that it is and why he did it? Also, can you explain how they became positive? Thanks!
    (12 votes)
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    • blobby green style avatar for user paullovelace
      ๐š ๐š‘๐šŽ๐š— ๐šข๐š˜๐šž ๐š๐šŠ๐šŒ๐š๐š˜๐š› ๐š˜๐šž๐š ๐šŠ ๐šœ๐š’๐š๐š—, ๐š๐š•๐š’๐š™ ๐š๐š‘๐šŽ ๐šœ๐š’๐š๐š—๐šœ. ๐šŽ๐šŸ๐šŠ๐š•๐šž๐šŠ๐š๐šŽ ๐š‹๐š˜๐š๐š‘ ๐šœ๐š’๐š๐šŽ๐šœ ๐š–๐šŠ๐š”๐š’๐š—๐š ๐šœ๐šž๐š›๐šŽ ๐š๐š‘๐šŽ ๐šŽ๐šš๐šž๐šŠ๐š๐š’๐š˜๐š— ๐š’๐šœ ๐šŒ๐š˜๐š›๐š›๐šŽ๐šŒ๐š. ๐šœ๐š˜๐š›๐š›๐šข ๐š’๐š ๐š๐š‘๐šŠ๐š ๐š๐š’๐š๐š—'๐š ๐š‘๐šŽ๐š•๐š™.
      (3 votes)
  • blobby green style avatar for user paullovelace
    ๐ป๐‘œ๐“Œ ๐’น๐‘œ ๐“Ž๐‘œ๐“Š ๐’ป๐’ถ๐’ธ๐“‰๐‘œ๐“‡ ๐‘œ๐“Š๐“‰ ๐’ถ ๐“ƒ๐‘’๐‘”๐’ถ๐“‰๐’พ๐“‹๐‘’ ๐“ˆ๐’พ๐‘”๐“ƒ?
    (10 votes)
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  • blobby blue style avatar for user pookie
    but why did he put it to adding and not subtracting instead?
    (5 votes)
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    • mr pink green style avatar for user David Severin
      While I would not have done it this way because I have a very good sense of adding and subtracting numbers, the reason Sal factored out a -, thus changing all the signs, is because he saw that the two negatives would give a bigger answer than the positive number, and at this point in 7th grade, more people would do better to subtract a smaller number from a larger number, then just add the negative sign. I would tend to add the two negatives (if you have same signs, you add and keep the sign), so this would give .79 - 137.2, then subtract (opposite signs subtract and take sign of largest absolute value number) to get -136.41.
      I really push learning these two addition/subtraction rules with my students over switching things around like Sal did.
      (6 votes)
  • duskpin seed style avatar for user Kayla Casimier
    i thought when you change a subtraction sign to a plus sign then the number behind it would become negitive? Please explain this to me because this video just confused me?
    (4 votes)
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  • duskpin ultimate style avatar for user Adrianna Crystalfield
    This is so confusing! Can someone explain it with this question? I'm having a bit of trouble with this one;

    -45/40 + 1.875=
    (5 votes)
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  • aqualine ultimate style avatar for user Rishon Sony
    im confused why did he turn 79% into .79?
    (4 votes)
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  • hopper happy style avatar for user Griff
    At he talks about "factoring out" a negative sign what exactly is this and what is it used for?
    (5 votes)
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  • starky tree style avatar for user ^-^
    How do you calculate an addition/subtraction equation that starts with a negitive decimal and the rest are postive decimals?
    (3 votes)
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    • leaf green style avatar for user hyeri389
      To give an example, are you talking about a question like this?
      -.325 + .585?
      If that is the case, then think of it like this. If the question is adding, and the negative decimal comes first, then do this. .585 -325. That should give you the answer.
      If you are multiplying, such as this,
      -.325 x .585
      it's the same thing as multiplying negative numbers. Multiplying a negative number by a positive number will get you a negative number. So, do this. Take the decimals out, and multiply. You should get this equation.
      -325 x 585= -190125.
      Now, since you took 6 decimal places out, put 6 in. Your final answer should be -.190125.
      If you are subtracting like this,
      -.325 - .585,
      that 585 will actually ADD to the negative -.325, making it -.910.
      If you are dividing, think of it as the same thing as multiplying. Good luck!
      (6 votes)

Video transcript

79% minus 79.1 minus 58 and 1/10. And I encourage you to pause this video and try to compute this expression on your own. Well, the thing that jumps out at you is that these are in different formats. This is a percentage. These are different representations. There's a percentage. This is a decimal. This is a mixed number. And so to make sense of it, it's probably a good idea to get them all in the same format. And it seems like we could get all of these into a decimal format pretty easily. So let's go that way. So 79%, that literally means 79 per 100. If you wanted to write it as a fraction, it would be 79/100. But if you wanted to write it as a decimal, it's 0.79, which could be 0-- or we would write it down a 0.79. Now, 79.1 is already written as a decimal, so we'll just write it again. So minus 79.1. And then, 58 and 1/10. Well, 1/10 is the same thing as 0.1. So you could view this as 58-- well, and literally as 1/10. So it's minus 58 and 1/10. Or, you could view this as 58.1. So now they're all in the same format, let's actually do the computation. Now, the first thing that jumps out at you is you have a fairly small number here. Small positive number. It's less than 1. And you're subtracting fairly large numbers over here. So your whole answer is going to be negative. And to make sense of this a little bit, what I'm going to do is I'm going to factor out a negative sign. And that'll make the computation-- at least in my brain, it's going to make it a little bit easier. So if we factor out a negative sign, this becomes-- so we're going to factor it out. Actually, let me just do it this way. So if we factor out a negative sign, then this will become negative. This would be positive. And this would be positive. And just to verify this, imagine distributing this negative sign, or if this was a negative 1. Negative 1 times this is positive. Negative 1 times this is negative. Negative 1 times this is negative. So these two expressions are the exact same thing. And the reason why I did that is now we'll do the more natural thing of we will add these two numbers. We'll get a positive number, a larger positive number than what we're going to subtract from it right over here. So we can use our traditional method. Although, we can't forget about this negative out here. So let's first do that. Let's add 79.1 plus 58.1. So 79.1 plus 58.1. So 0.1 plus a 0.1 is 0.2. 9 plus 8 is 17. So that's seven 1's and one 10. So one 10 plus seven 10's is going to get us to 0.8. Plus 0.5 gets us to thirteen 10's, or 130. So we have 137.2 is this part right over here. So 100. Let me write this down. So we have 137.2. And then from that, adding a negative 0.79 is equivalent of subtracting 0.79. So let's do that. Let's subtract 0.79, making a point to align our decimal points so that we're subtracting the right place from the right place. And now let's do our subtraction. So right now we're subtracting 9 from nothing. We could write a 0 right over here, but we still face an issue in the hundredths place. We're also subtracting a 0.7 from 0.2. So we're going to have to regroup a little bit in the numerator in order to subtract. Or at least, in order to subtract using the most traditional technique. So let's take a tenth from the 2, so it's only one tenth now, and give it to the hundredths. So one tenth is ten hundredths. So we could subtract that ten hundredths minus nine hundredths is one hundredth. Now in the tenths place. We don't have enough up here, so let's take 1 from the one's place. So that becomes a 6. 1 is ten tenths. So now we have 11 tenths. 11 minus 7 is 4. Add our decimal place. 6 minus 0 is 6. And then we got our 13 just like that. So outside the parentheses, I still have the negative sign. When I computed all of this inside the parentheses, I got 136.41. And then we can't forget about the negative sign out here. So this whole thing computes to negative 136.41.