The arithmetic pattern of adding 10 to a number will help us find the sum of 9+8.
We know that 4+10 is 14, 6+10 is 16 or 2+10 is 12. You can always add 9 to something by adding 10 to the number (instead of 9) and then counting down one (since 9 is one less than 10). For example: to find 9+8, add 10+8 (10 instead of 9), which is 18, and then count down one to get 17.
The same pattern goes with adding 8 to a number, you can add 10, and then count down two numbers. So, to find the sum of 9+8, add 9+10 which is 19, and then counting down two numbers, the answer is 17.
Find out why young kids are struggling with Common Core math at the Washington Post.