Adding and taking away within 18 · Usually met around P1
Adding 9 and 8
+9: add 10, take 1 back (+8: take 2).
How to do it
Method
+9: add 10, take 1 back (+8: take 2)
7 + 9 = 7 + 10 − 1 = 16
Another way
Make ten
9 + 1 = 10, 10 + 6 = 16
In full
Adding 9 is adding 10, then taking 1 away. 7 + 9 → 17 − 1 = 16. Adding 8 is adding 10, then taking 2 away: 7 + 8 → 17 − 2 = 15. This is called compensation. Or make ten: in 7 + 9, the 9 is 1 short of 10, so split 7 into 1 and 6; 9 + 1 = 10, 10 + 6 = 16. The same answer.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
9 + 8
Show the answer and working
Answer: 9 + 8 = 17
Adding 8 is adding 10 and taking 2 back: 9 + 10 = 19, 19 − 2 = 17
Example 2
8 + 8
Show the answer and working
Answer: 8 + 8 = 16
Adding 8 is adding 10 and taking 2 back: 8 + 10 = 18, 18 − 2 = 16
Example 3
7 + 9
Show the answer and working
Answer: 7 + 9 = 16
Adding 9 is adding 10 and taking 1 back: 7 + 10 = 17, 17 − 1 = 16
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