This strategy is harder for students to understand. There isn't really a "trick" or a fast way to teach this strategy.
I have taught the students to think of addition in order to solve these problems.
Example: 15-8
The students know to think of 8 + ___ = 15. If they know their addition facts, they should easily be able to say the answer is 7. So 15-8=7.
Another way I taught them is more confusing for some students, but it does work for a few of them.
I will use the same example as above to show you this way:
- Look at the problem and determine if it is a near doubles problem. To do this, double the second number in the problem. Example - 15-8=____. Double the 8 which is 8+8=16. Since the answer 16 is one away from the 15 in the original problem, the near doubles strategy can be used.
- You've already doubled 8, so you'll use that for the next step. 15 is one less than 16, so you have to take one away from one of the 8s. Then you get 8+7=15.
- In this case, the answer when you doubled was greater than the number in the original problem, so you have to take away one. In other cases, the answer when you double the number is less than the number in the original problem, so you would add one to the numbers.
Even though I wrote out the steps on the board, all this work is supposed to be done in their heads for this mental math strategy. I wrote out the steps to teach them how to visualize it.
Want a video resource?
Here is a LearnZillion video that shows how addition and subtraction using near doubles are related.
If you just want the LearnZillion quick code:
LZ3737
As always...
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