# Mental math double plus 1 strategy

Adding doubles is a mental math double plus 1 strategy that can be used to quickly add numbers. To use this strategy, you take two numbers that are the same and add them together. For example, if you’re adding 5 + 5, you would double 5 to get 10.
Eventually students will memorize these facts, which is expected by the end of grade 2. “Double plus one” facts are based on knowing doubles first, then just counting on one. This allows students to answer questions in which two adjacent numbers, such as 4 and 5, are added: Double 4 = 8, so 4 + 5 = 8 + 1 = 9.

## What Are Mental math double plus 1 strategy ?

Using a Mental math double plus 1 strategy-based approach to adding and subtracting helps to master addition and subtraction facts. One such strategy we use to understand addition or subtraction facts is the doubles strategy or fact.

4 + 4 = 8  is a doubles fact. Doubles fact can help us know other additional strategies like doubles plus one.  This strategy is used in the addition of two consecutive numbers, like 2 + 3. Here, we break one of the addends to make a double with the other addend.

When we add two of the same number, we add using the “doubles fact”.

For example, 1 + 1 and 2 + 2 are both doubles facts.

### WHAT IS A NEAR Mental math double plus 1 strategy?

near double is a math fact that is close to a doubles fact. For example, 6+7 is considered a near double because it is close to the doubles fact 6+6.

Near doubles could be doubles plus one facts, doubles plus two facts, or doubles minus one facts.

Let’s take a look at the doubles fact 5+5. 5+6 would be the double plus one (5+5 plus one more). 5+7 would be the double plus two (5+5 plus two more). 5+4 would be the double minus one (5+5 subtract one).

What is the mental strategy for addition?
• Add the tens digits and then look at the ones digits to decide what the answer ends in.
• For example in 32 + 29, we add the tens digits first.
• 3 + 2 = 5.
• Adding the ones, 2 + 9 = 11

## How to Add Numbers with Mental Math Doubles Plus One Method

Knowing doubles facts lays a foundation for solving doubles plus one addition. Let’s begin by adding two consecutive numbers using this method.

For example, consecutive numbers 8 and 9 have been added using the strategy.

We know that 8 + 8 is 16, so 8 + 9  = 17

Let’s look at another example of this strategy. Use the image to solve 4 + 5.

We know that 4 + 4 = 8, so 4 + 5 = 4 + 4 + 1 = 8 + 1 = 9

## Fun Fact!

Doubles plus 1 and doubles minus one are called “near doubles strategies”.

## Solved Examples

Solution: Using the doubles plus 1 method:

4 + 3 = 3 + 1+ 3

6 + 7 = 6 + 6 + 1

Example 2: Add using the near doubles fact.

Solution:  Since 5 + 5 = 10 , 5 + 6 is one more than 5 + 5.

So, 5 + 6 = 5 + 5 + 1 = 11

Example 3: Solve the following equations using doubles plus 1:

1. 101 + 100 = ?
2. 40 + 41 = ?

Solution:

1. Since we know 100 + 100 = 200,

101 + 100 = 100 + 1 + 100 = 201

1. Since we know 40 + 40 = 80,

40 + 41 = 40 + 40 + 1 = 81

4 + 3 + 1
8 + 4
8 + 1
8 + 5

3 + 3 = 6
4 + 4 = 8
5 + 5 = 10
6 + 6 = 12