More Mental Math: Doubles Plus One

Using Doubles Plus One to Solve Addition

If you recall your years in elementary school, perhaps one of your most hated activities was memorizing math facts.  Whether it was through flash cards or “fun” games, it seems we could never get away from these facts.  Common Core math utilizes a strategy that can help make that memorization a little easier.  I don’t know about you, but “doubles” were always some of the facts that stuck with me.  For example, 2+2=4, 9+9 =18, etc.  The Doubles Plus One strategy can help a student take just a few facts (the doubles) and expand that to many more facts.  When you are first introduced to this idea it might seen confusing, or like you are taking extra steps to find an already simple solution.  Let’s take a look at the process you might see on a homework problem your child brings home.

Example 1 – Starting Simple

Show how to use a doubles fact to solve 3+2.  OK, first off, you might think, my child already knows 3+2!  That is a simple math fact, and if they don’t know it they can easily solve it on their fingers.  Remember, we often start with simple problems to show how a strategy works.  We also start by writing a lot of it on paper to see the steps, when ultimately we want kids to do it quickly in their head.  This is part of the learning process!  Now to complicate it even more, let’s say you see a weird diagram that looks like this:

Doubles Plus One 3 2

Now we have taken a simple problem of 3+2 and added more confusion!  Ultimately, this is to teach the process so it will hopefully become second nature mental math.  The smaller 2 and 1 that you see coming down from the 3 is a number bond.  If you are unfamiliar with this term, be sure to review my previous post.  So let’s bring it back to the beginning.  We are learning the strategy of Doubles Plus One.  Even if your child already knows 3+2, they can see how to apply the Doubles Plus One concept.  We start by breaking apart 3 into 1 and 2.  We can then use the doubles fact of 2+2 to solve this problem.  Next, you take the total of 4 and just add one, getting your answer of 5.

Doubles Plus One 2 2

Example 2 – Time for a Challenge

Once your child understands the concept, try some harder ones.  Can they do 8+9?

Doubles Plus One 8 9

Notice that the number bond is coming off of the 9, so they should be thinking of 8 and 1 to complete this problem.  Next they can use their doubles fact for 8 then just add 1 to find the answer.  Once your child has it down, try doing some mental math problems.  If they get stuck, write it down and use the number bond to help them find the doubles fact.

Expanding the Concept

Does Doubles Plus One seem like a piece of cake now?  The great thing about this strategy is that it can go even further.  What about Doubles Plus Two?  You could even try Doubles Minus One or Doubles Minus Two.  The cool thing about Common Core math strategies is that this is something you probably already do in your head.  I know I do. But I don’t remember ever being taught to do this.  How much more successful would I have been at math if I had been taught this strategy from 1st or 2nd grade?  Let me know what you think!  Is this a useful tool in your child’s math toolbox?

Common Core Math Standards

1st Grade:

CCSS.MATH.CONTENT.1.OA.C.6
Add and subtract within 20, demonstrating fluency for addition and subtraction within 10. Use strategies such as counting on; making ten (e.g., 8 + 6 = 8 + 2 + 4 = 10 + 4 = 14); decomposing a number leading to a ten (e.g., 13 – 4 = 13 – 3 – 1 = 10 – 1 = 9); using the relationship between addition and subtraction (e.g., knowing that 8 + 4 = 12, one knows 12 – 8 = 4); and creating equivalent but easier or known sums (e.g., adding 6 + 7 by creating the known equivalent 6 + 6 + 1 = 12 + 1 = 13).

2nd Grade:

CCSS.MATH.CONTENT.2.OA.B.2
Fluently add and subtract within 20 using mental strategies. By end of Grade 2, know from memory all sums of two one-digit numbers.

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