Video: Skip Counting by 10s to 120

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

03:28

Video Transcript

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.

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