Lesson Video: Counting by Tens up to 120 | Nagwa Lesson Video: Counting by Tens up to 120 | Nagwa

Lesson Video: Counting by Tens up to 120 Mathematics

In this video, we will learn how to skip count forward or backward in tens from any starting number within 120.


Video Transcript

Counting by 10s up to 120

In this video, we’re going to learn how to skip count in 10s. We’re going to do this from any starting number up to 120. And as well as counting forwards, we’re also going to skip count backwards.

You recognize this, don’t you? It’s a 10s block. One 10 on its own is worth 10. Now, we have two 10s or 20. Let’s continue skip counting in 10s from 20: 30, 40, 50. Can you see how these numbers are changing? If we make these numbers out of a tens card and a ones card, perhaps you’ll be able to see what’s happening. 10, 20, 30, 40, 50. It’s the number of 10s that changes every time. We didn’t have to touch our ones card at all. The number of ones stays the same. Now, we’ve run out of space for our 10s blocks, so let’s find some other ways to skip count in 10s.

Number lines can be useful for helping showing skip counting, and 100 squares are really helpful. Let’s carry on skip counting from 50: 60, 70, 80. Can you see what’s happening here? Each time we count 10 more, we move to the next square down in the 100 square. 80, 90, 100. But, you know, we don’t even have to stop there. 100 squares are really helpful, but numbers don’t just stop at 100, do they? Let’s do something dangerous. Let’s break out of our 100 square and keep counting: 100, 110, 120. We could carry on, but this time we really will stop. So that’s counting forwards in 10s.

And counting backwards in 10s simply means going in the opposite direction. So, say the number we started with was 120, we could count back in 10s by moving up the 100 square. 110, 100, 90, 80, 70, 60, and so on. So far, all the numbers we’ve been talking about have been multiples of 10. These are numbers that we can make from a number of 10s but don’t have any ones. So numbers like 50 and 30 and 20 are all multiples of 10 because they don’t contain any ones. They all end in a zero. Did you notice that?

But at the start of the video, that if you remember, we said we were going to learn how to skip count in 10s from any number up to 120, not just all the multiples of 10. We could skip count from 53 or 27. Don’t know if you remember that title page of the video, but there were two children on there; they were skip counting from 35. What happens when we skip count in 10s from any number up to 120? Let’s choose a two-digit number to start with. How about 73? 73 is made up of seven 10s and three ones. We know it’s not a multiple of 10 this time because it doesn’t end in a zero.

So what’s going to happen if we skip count in 10s from 73? We know that as we skip count forwards in 10s, we get one more 10 every time. So instead of seven 10s, we’re now going to have eight 10s. Instead of 73, we now have 83. And look where we are on our 100 square. We’ve moved down again, just like we did when we were counting in multiples of 10. 10 more is just one square down on a 100 square. And from eight 10s, we can count on to nine 10s. When we count forwards in 10s, the number of 10s increases, but the number of ones stays the same. Did you notice how all our numbers ended in the digit three? 73, 83, 93.

Now, if we start with 73 and count backwards in 10s, the number of 10s is going to get smaller. When we count backwards in 10s, the number of 10s decreases, but the number of ones still stays the same. So, skip counting in 10s is a really quick way to count. But it’s all about changing the number of 10s and keeping the number of ones the same. Let’s try answering some questions now where we need to put into practice our skip-counting skills.

Subtract 10 each time when skip counting backward by 10s. Fill in the next three numbers.

Can you see what we have in the picture here? It’s a number line, but it’s a number line with a difference. You might be used to number lines that go from left to right, and we read along them. Well, in the picture, we can see a number line that goes up and down, and we’re going to need to read it looking up and down. We can see that there are some numbers at the top of this number line. So let’s start at the top and read down. 120, 110, 100. And then we have three empty boxes. And we can see at the bottom of the question that we’re going to need to fill in these next three numbers.

To do that, we need to understand what’s happening on our number line. In the first sentence, we’re told how to find the answer. We need to skip count backward by 10s. If we look at the other side of our number line, we can see some green arrows that say subtract 10, subtract 10. So what’s happening as we’re going from 120 to 110 to 100 is we’re taking away 10 or counting backward in 10 every time. So, to fill in the next three numbers, we need to carry on skip counting backward by 10s. But we’re going to start with a number that we got to, which was 100.

Now, 100 is the same as 10 lots of 10. As we skip count backwards in 10s, we need to take away one more lot of 10 every time. So instead of 10 10s, we’re going to have nine 10s. And nine 10s are worth 90. If we subtract another lot of 10, we’re going to go from nine 10s to eight 10s, and 90 becomes 80. And if we subtract one more 10 from 80, we’re going to have 70. We carried on skip counting backward by 10s by subtracting 10 every time: 120, 110, 100, 90, 80, 70. The missing three numbers in our pattern were 90, 80, and then 70.

A class are skip counting. 47, 57, what, 77, what. What number should Scarlett say? And what number should Michael say?

In this problem, we’re told that a class are skip counting. You know what skip counting is, don’t you? It’s where we make jumps of a certain number. Rather than counting in ones which can often be quite slow, we can skip count in another number and go a little bit quicker, make jumps of that number. Now, although we can see some of the numbers in this pattern, there are two missing. Both Scarlett and Michael’s speech bubbles are blank. And that’s why the two questions we’re being asked is which number should Scarlett say and which number should Michael say.

We need to use the parts of the sequence or pattern that we can see to help us find out the bits that we can’t see. To begin with, let’s write down the numbers that we know going down the page. We’ve got 47 then 57 and, obviously, Scarlett’s number that we don’t know. But then we do know 77. We end with Michael’s number that we don’t know. What do you notice about these numbers? Can you see that they all contain the same number of ones? They all end in a seven: 47, 57, and then later on we have 77.

But if we look at the number of 10s in each number, the only numbers we can really look at are these two that are next to each other. And we can see that the number of 10s goes up by one. 47 plus 10 equals 57. We know when we add a 10 to a number, the number of 10s increases because we’ve got one more 10, but the number of ones stays exactly the same. So, all our numbers in this pattern are going to have the same number of ones. They’re all going to end in a seven. Let’s put the seven ones in for our two missing numbers then.

Now, the class have said 47 and then 57 and then Scarlett says her number. We know that 57 has five 10s. And because she’s skip counting in 10s, Scarlett needs to add another 10. Instead of five 10s, her number is going to have six 10s: 47, 57, 67. And we can see why 77 follows on perfectly from this. Michael says that number that comes after 77. And 77 contains seven 10s. So if Michael adds one more 10 as he skip counts in 10s, his number is going to have eight 10s. Michael’s number is 87.

You know, when we skip count in certain numbers, we can actually hear the pattern as we say the numbers. And this is true when we skip count in 10s. You’ll be able to hear the pattern as we read these numbers: 47, 57, 67, 77, 87. We’ve found the missing numbers by skip counting in 10s. The number that Scarlett should say is 67. And the number that Michael should say is 87.

Olivia started at 11 and counted in 10s until she reached 41. Draw this on a number line. How many jumps of 10 did you make?

When we count in ones, we usually just call it counting. But if we ever count in jumps of another number, we call it skip counting. In this question, Olivia isn’t counting in ones; she’s counting in 10s. So we can say she’s skip counting in 10s. We’re told that she started at 11, and she counted in 10s until she reached 41. Now, normally, counting from 11 to 41 might take a little while. But because Olivia is skip counting in 10s, she’s going to get there quicker.

In the first part of the question, we’re asked to draw what Olivia does on a number line. So let’s do exactly what she does. She starts at 11 and counts on in 10s. Now, we know that number 11 is made up of one 10 and one one. So, as we count on in 10s, we need to add one more 10 every time. 11, 21, 31, 41. Did you notice how the number of ones didn’t change as we added 10? 11, 21, 31, 41. Now that we’ve completed that number line, we can finish off answering the question because we’re asked, how many jumps of 10 did we make? We made three jumps of 10. If Olivia starts at 11 and counts in 10s until she reaches 41, she needs to make three jumps of 10: 11, 21, 31, 41.

Complete the number pattern: 79, 69, what, what, 39.

In this problem, we’re given a number pattern that contains five numbers. Now, we know three of them; that’s the first two and the last one. But there are two in the middle that we don’t know, and we need to complete the pattern. What do you notice about the numbers that we do know? We’ve got 79, 69, and then, at the end, 39. They all contain nine ones, don’t they? As we move along the pattern, the number of ones doesn’t change. But if we look at the number of 10s, we can see that this does change. We go from seven 10s in 79 to six 10s in 69. The pattern decreases by 10.

This pattern seems to be that we’re skip counting backwards in 10s, so we need to take away 10 more each time. 69 has six 10s, so if we take away 10 from this, the next number will have five 10s. The next number in the pattern then is 59. 59 has five 10s. So if we take away 10 from this, instead of five 10s, we’re now going to have four 10s. 59 becomes 49. And we can tell that our pattern works because if we skip count backwards from 49, we reach 39. To complete the number pattern, we needed to skip count backwards in 10s. 79, 69, 59, 49, 39. The two missing numbers are 59 and 49.

What have we learned in this video? We’ve learned how to skip count in 10s, forward or backward, from any starting number up to 120. We’ve found that as we add or subtract 10s, the number of ones stays the same.

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