47
The numbers 17 and 18
Textbook Part 2, pp. 14–15
GoalBonds of 17 and 18; “gaps between” problems.
New words
seventeen · o‘n yettieighteen · o‘n sakkizin between · orasiga
Explanation
17 = 10 + 7 = 9 + 8. 18 = 10 + 8 = 9 + 9. Careful: if there are 9 trees in a row, there are 8 gaps between them (always one fewer). Drawing helps with problems like this.
Worked examples
9 maples grow along a road. One fir goes between each pair: 8 firs. In total: 9 + 8 = 17 trees.
9 + 9 = 18
Class activity
5 children stand in a row. How many children fit in the gaps between them? Children check for themselves (4 gaps).
Practice
1
9 + 8 = ?
17
2
18 − 9 = ?
9
3
10 + _ = 18
8
4
There are 6 posts in a row. How many gaps between them?
5