1.2 Add whole numbers  (Page 12/6)

Use addition notation

A college student has a part-time job. Last week he worked $3$ hours on Monday and $4$ hours on Friday. To find the total number of hours he worked last week, he added $3$ and $4.$

The operation of addition combines numbers to get a sum    . The notation we use to find the sum of $3$ and $4$ is:

We read this as three plus four and the result is the sum of three and four. The numbers $3$ and $4$ are called the addends. A math statement that includes numbers and operations is called an expression.

Model addition of whole numbers

Addition is really just counting. We will model addition with $\text{base-10}$ blocks. Remember, a block represents $1$ and a rod represents $10.$ Let’s start by modeling the addition expression we just considered, $3+4.$

Each addend is less than $10,$ so we can use ones blocks.

We start by modeling the first number with 3 blocks.
Then we model the second number with 4 blocks.
Count the total number of blocks.

There are $7$ blocks in all. We use an equal sign $\text{(=)}$ to show the sum. A math sentence that shows that two expressions are equal is called an equation. We have shown that. $3+4=7.$

Model the addition $2+6.$

Solution

$2+6$ means the sum of $2$ and $6$

Each addend is less than 10, so we can use ones blocks.

Model the first number with 2 blocks.
Model the second number with 6 blocks.
Count the total number of blocks	There are $8$ blocks in all, so $2+6=8.$

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When the result is $10$ or more ones blocks, we will exchange the $10$ blocks for one rod.

Model the addition $5+8.$

Solution

$5+8$ means the sum of $5$ and $8.$

Each addend is less than 10, se we can use ones blocks.
Model the first number with 5 blocks.
Model the second number with 8 blocks.
Count the result. There are more than 10 blocks so we exchange 10 ones blocks for 1 tens rod.
Now we have 1 ten and 3 ones, which is 13.	5 + 8 = 13

Notice that we can describe the models as ones blocks and tens rods, or we can simply say ones and tens . From now on, we will use the shorter version but keep in mind that they mean the same thing.

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