Video: Count Forward from a Given Number within the Known Sequence

Which number is missing? 17, 18, 19, _, 21.

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Video Transcript

Which number is missing? 17, 18, 19, the missing number, 21.

We need to find the number which comes between 19 and 21, or the number that comes after 19 but before 21. We could use a hundred square to help us find the missing number. We’ve circled number 19. So which number comes after 19? 20. The missing number is 20. 17, 18, 19, 20, 21.

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