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Subtraction word problem: crayons

Sal solves a subtraction word problem with numbers less than 100.

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  • purple pi teal style avatar for user Noahjrkingsb
    Can't it be 64 - 31 instead of 64 - _ =31?
    (8 votes)
  • aqualine sapling style avatar for user buttonboxx2321
    How do you know what number should go first when subtracting adding multiplying and dividing?
    (3 votes)
  • piceratops ultimate style avatar for user spiralofhope
    -- Why is he able to move the number on the right just because he feels like it?

    Oh, I guess I see it in pictures, but it's still confusing =( I'll try more.
    (0 votes)
    • male robot hal style avatar for user authentic8
      It is just a different way of writing the same equation. The rule with equations is that the value both sides of the '=' sign must always be equal even if we move the numbers around (that is why it is called an "equation"). As long as the equality still exists, it is OK to move the numbers in an equation around anywhere you like. In practice, this means if you move a positive number from one side of the equation to the other you must change it to make it negative, and if you move a negative number from one side of the equation you must make it positive.

      In this case, as the unknown value represented by '?' is the value we want to find out, it makes sense to put it on one side of the equation (usually the left by convention) and all the other known values 64 and 31 on the other side. This makes it easier to work out the missing value on the left. Since we can now easily work out the value on the right side, as both sides of an equation must be equal, we will also know what the value is on the left side.

      Hope that helps!
      (6 votes)

Video transcript

- [Voiceover] Mrs. Henry's class has 64 crayons. At the end of the year only 31 crayons remain. How many crayons have been used? And they give us a picture here. So, the beginning of the year, this is 64 crayons. Six tens and four ones. One, two, three, four, five, six tens, and one, two, three, four ones. So this is 64 at the beginning of the year, and at the end of the year only 31 crayons remain. Well, 31 is three tens and one one. Three tens. One ten, two tens, three tens, and one one. So how many have been used? And try to pause the video and try it on your own. All right, I assume you have tried it. Let's do it together. So there's a couple of ways to do it. If we didn't have the picture here, we would say all right, she started, her class started with 64 crayons, they used some, so if we are using crayons that would kind of subtract the crayons away. So if you're using some crayons, you would subtract some number from 64, so I'm just gonna put a blank here. So 64 minus the number that are used is going to be equal to the number that remain. So this is going to be equal to what's left over, and they tell us that 31 crayons remain. 31 crayons remain. So, we need to figure out what's here? What is this blank? 64 minus blank is 31. Well, if 64 minus blank is 31, then I can swap the 31 and the blank. That means that 64 minus 31 is going to be equal to blank. That means that 64 minus 31 is going to be equal to blank. How did I do that? How do I think about that? Well, imagine 64 things. So if this block represents 64. So this is 64, just like that, and if this is 31, and then this is our blank. We know that the blank, so I could put a question mark here if I like. I could put a question mark. So this is the question mark. So we know that 64 minus the question mark would be 31. Or that 31 plus question mark would be 64. Or that 64 minus 31 is going to get us the question mark right over here. So we just have to figure out what 64 minus 31 is. And let's do that. Let's do that right over here. So 64 minus 31, and it's going to be equal to, so let's look at that ones place. Four ones minus one one is going to be three ones. Six tens minus three tens is three tens. So the blank is 33. And you could check that, 64 minus 33 is 31. Or 64 minus 31 is 33. So 33 crayons have been used. Now since they made this neat picture for us, we actually could have just used the picture. How would we have done that? Well, this is what they started with. This is the 64. Six tens. One, two, three, four, five, six tens, and one, two, three, four ones. So 64, and they end up with 31. So how many of the tens do we use up to go from what we started with to what we finish with? Well, we end up using, let's see, there's only three tens here. There were six tens here. So we used one, two, three tens. So we used up three tens. And how many ones do we use up? Well, we have four ones here, and we only have one right over here. So we use up three ones. We used up three ones. If you take away these three tens, and these three ones, you'll be left with the same number of crayons that we have at the end of the year. So we use up three, I guess you could say, we used up three ten packs, and we used up three ones. So what's that going to be? Well, three tens plus three ones is 33.