Video: Working with Number Patterns Involving Numbers up to 99

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


Video Transcript

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.

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