# 1.4: Multiply Whole Numbers (Part 1)

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)

\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)

Skills to Develop

- Use multiplication notation
- Model multiplication of whole numbers
- Multiply whole numbers
- Translate word phrases to math notation
- Multiply whole numbers in applications

be prepared!

Before you get started, take this readiness quiz.

- Add: 1,683 + 479. If you missed this problem, review Example 1.21.
- Subtract: 605 − 321. If you missed this problem, review Example 1.33.

### Use Multiplication Notation

Suppose you were asked to count all these pennies shown in Figure 1.11.

*Figure 1.11*

Would you count the pennies individually? Or would you count the number of pennies in each row and add that number 3 times.

$$8 + 8 + 8$$

Multiplication is a way to represent repeated addition. So instead of adding 8 three times, we could write a multiplication expression.

$$3 \times 8$$

We call each number being multiplied a factor and the result the **product**. We read 3 × 8 as *three times eight*, and the result as *the product of three and eight*.

There are several symbols that represent multiplication. These include the symbol × as well as the dot, • , and parentheses ( ).

Operation Symbols for Multiplication

To describe multiplication, we can use symbols and words.

Operation | Notation | Expression | Read as | Result |
---|---|---|---|---|

Multiplication | × | 3 × 8 | three times eight | the product of 3 and 8 |

• | 3 • 8 | |||

() | 3(8) |

Example 1.39:

Translate from math notation to words: (a) 7 × 6 (b) 12 · 14 (c) 6(13)

##### Solution

(a) We read this as *seven times six* and the result is *the product of seven and six*.

(b) We read this as *twelve times fourteen* and the result is *the product of twelve and fourteen*.

(c) We read this as *six times thirteen* and the result is *the product of six and thirteen*.

Exercise 1.77:

Translate from math notation to words: (a) 8 × 7 (b) 18 • 11

Exercise 1.78:

Translate from math notation to words: (a) (13)(7) (b) 5(16)

### Model Multiplication of Whole Numbers

There are many ways to model multiplication. Unlike in the previous sections where we used base-10 blocks, here we will use counters to help us understand the meaning of multiplication. A counter is any object that can be used for counting. We will use round blue counters.

Example 1.40:

Model: 3 × 8.

##### Solution

To model the product 3 × 8, we’ll start with a row of 8 counters.

The other factor is 3, so we’ll make 3 rows of 8 counters.

Now we can count the result. There are 24 counters in all.

$$3 \times 8 = 24$$

If you look at the counters sideways, you’ll see that we could have also made 8 rows of 3 counters. The product would have been the same. We’ll get back to this idea later.

Exercise 1.79:

Model each multiplication: 4 × 6.

Exercise 1.80:

Model each multiplication: 5 × 7.

### Multiply Whole Numbers

In order to multiply without using models, you need to know all the one digit multiplication facts. Make sure you know them fluently before proceeding in this section. Table 1.39 shows the multiplication facts. Each box shows the product of the number down the left column and the number across the top row. If you are unsure about a product, model it. It is important that you memorize any number facts you do not already know so you will be ready to multiply larger numbers.

#### Table 1.39

× | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
---|---|---|---|---|---|---|---|---|---|---|

0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |

1 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |

2 | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 |

3 | 0 | 3 | 6 | 9 | 12 | 15 | 18 | 21 | 24 | 27 |

4 | 0 | 4 | 8 | 12 | 16 | 20 | 24 | 28 | 32 | 36 |

5 | 0 | 5 | 10 | 15 | 20 | 25 | 30 | 35 | 40 | 45 |

6 | 0 | 6 | 12 | 18 | 24 | 30 | 36 | 42 | 48 | 54 |

7 | 0 | 7 | 14 | 21 | 28 | 35 | 42 | 49 | 56 | 63 |

8 | 0 | 8 | 16 | 24 | 32 | 40 | 48 | 56 | 64 | 72 |

9 | 0 | 9 | 18 | 27 | 36 | 45 | 54 | 63 | 72 | 81 |

What happens when you multiply a number by zero? You can see that the product of any number and zero is zero. This is called the Multiplication Property of Zero.

Definition: Multiplication Property of Zero

The product of any number and 0 is 0.

$$a \cdot 0 = 0$$

$$0 \cdot a = 0$$

Example 1.41:

Multiply: (a) 0 • 11 (b) (42)0

##### Solution

(a)

The product of any number and zero is zero. | 0 • 11 = 0 |

(b)

Multiplying by zero results in zero. | (42)0 = 0 |

Exercise 1.81:

Find each product: (a) 0 • 19 (b) (39)0

Exercise 1.82:

Find each product: (a) 0 • 24 (b) (57)0

What happens when you multiply a number by one? Multiplying a number by one does not change its value. We call this fact the Identity Property of Multiplication, and 1 is called the multiplicative identity.

Definition: Identity Property of Multiplication

The product of any number and 1 is the number.

$$1 \cdot a = a$$

$$a \cdot 1 = a$$

Example 1.42:

Multiply: (a) (11)1 (b) 1 • 42

##### Solution

(a)

The product of any number and one is the number. | (11)1 = 11 |

(b)

Multiplying by one does not change the value. | 1 • 42 = 42 |

Exercise 1.83:

Find each product: (a) (19)1 (b) 1 • 39

Exercise 1.84:

Find each product: (a) (24)(1) (b) 1 × 57

Earlier in this chapter, we learned that the Commutative Property of Addition states that changing the order of addition does not change the sum. We saw that 8 + 9 = 17 is the same as 9 + 8 = 17.

Is this also true for multiplication? Let’s look at a few pairs of factors.

$$\begin{split} 4 \cdot 7 & = 28 \qquad 7 \cdot 4 = 28 \\ 9 \cdot 7 & = 63 \qquad 7 \cdot 9 = 63 \\ 8 \cdot 9 & = 72 \qquad 9 \cdot 8 = 72 \end{split}$$

When the order of the factors is reversed, the product does not change. This is called the Commutative Property of Multiplication.

Definition: Commutative Property of Multiplication

Changing the order of the factors does not change their product.

$$a \cdot b = b \cdot a$$

Example 1.43:

Multiply: (a) 8 • 7 (b) 7 • 8

##### Solution

(a)

Multiply. | 8 • 7 = 56 |

(b)

Multiply. | 7 • 8 = 56 |

Changing the order of the factors does not change the product.

Exercise 1.85:

Multiply: (a) 9 • 6 (b) 6 • 9

Exercise 1.86:

Multiply: (a) 8 • 6 (b) 6 • 8

To multiply numbers with more than one digit, it is usually easier to write the numbers vertically in columns just as we did for addition and subtraction.

We start by multiplying 3 by 7.

$$3 \times 7 = 21$$

We write the 1 in the ones place of the product. We carry the 2 tens by writing 2 above the tens place.

Then we multiply the 3 by the 2, and add the 2 above the tens place to the product. So 3 × 2 = 6, and 6 + 2 = 8. Write the 8 in the tens place of the product.

The product is 81.

When we multiply two numbers with a different number of digits, it’s usually easier to write the smaller number on the bottom. You could write it the other way, too, but this way is easier to work with.

Multiply: 15 • 4.

##### Solution

Write the numbers so the digits 5 and 4 line up vertically. | |

Multiply 4 by the digit in the ones place of 15. 4 • 5 = 20. | |

Write 0 in the ones place of the product and carry the 2 tens. | |

Multiply 4 by the digit in the tens place of 15. 4 ⋅ 1 = 4. Add the 2 tens we carried. 4 + 2 = 6. | |

Write the 6 in the tens place of the product. |

Exercise 1.87:

Multiply: 64 • 8.

Exercise 1.88:

Multiply: 57 • 6.

Multiply: 286 • 5.

##### Solution

Write the numbers so the digits 5 and 6 line up vertically. | |

Multiply 5 by the digit in the ones place of 286. 5 • 6 = 30. | |

Write the 0 in the ones place of the product and carry the 3 to the tens place. Multiply 5 by the digit in the tens place of 286. 5 • 8 = 40. | |

Add the 3 tens we carried to get 40 + 3 = 43. Write the 3 in the tens place of the product and carry the 4 to the hundreds place. | |

Multiply 5 by the digit in the hundreds place of 286. 5 • 2 = 10. Add the 4 hundreds we carried to get 10 + 4 = 14. Write the 4 in the hundreds place of the product and the 1 to the thousands place. |

Exercise 1.89:

Multiply: 347 • 5.

Exercise 1.90:

Multiply: 462 • 7.

When we multiply by a number with two or more digits, we multiply by each of the digits separately, working from right to left. Each separate product of the digits is called a partial product. When we write partial products, we must make sure to line up the place values.

HOW TO: MULTIPLY TWO WHOLE NUMBERS TO FIND THE PRODUCT

Step 1. Write the numbers so each place value lines up vertically.

Step 2. Multiply the digits in each place value.

- Work from right to left, starting with the ones place in the bottom number.
- Multiply the bottom number by the ones digit in the top number, then by the tens digit, and so on.
- If a product in a place value is more than 9, carry to the next place value.
- Write the partial products, lining up the digits in the place values with the numbers above.

- Repeat for the tens place in the bottom number, the hundreds place, and so on.
- Insert a zero as a placeholder with each additional partial product.

Step 3. Add the partial products.

Example 1.46:

Multiply: 62(87).

##### Solution

Write the numbers so each place lines up vertically | |

Start by multiplying 7 by 62. Multiply 7 by the digit in the ones place of 62. 7 • 2 = 14. Write the 4 in the ones place of the product and carry the 1 to the tens place. | |

Multiply 7 by the digit in the tens place of 62. 7 • 6 = 42. Add the 1 ten we carried. 42 + 1 = 43. Write the 3 in the tens place of the product and the 4 in the hundreds place. | |

The first partial product is 434. | |

Now, write a 0 under the 4 in the ones place of the next partial product as a placeholder since we now multiply the digit in the tens place of 87 by 62. Multiply 8 by the digit in the ones place of 62. 8 • 2 = 16. Write the 6 in the next place of the product, which is the tens place. Carry the 1 to the tens place. | |

Multiply 8 by 6, the digit in the tens place of 62, then add the 1 ten we carried to get 49. Write the 9 in the hundreds place of the product and the 4 in the thousands place. | |

The second partial product is 4960. Add the partial products. |

The product is 5,394.

Exercise 1.91:

Multiply: 43(78).

Exercise 1.92:

Multiply: 64(59).

Example 1.47:

Multiply: (a) 47 • 10 (b) 47 • 100.

##### Solution

(a) 47 • 10 | |

(b) 47 • 100 |

When we multiplied 47 times 10, the product was 470. Notice that 10 has one zero, and we put one zero after 47 to get the product. When we multiplied 47 times 100, the product was 4,700. Notice that 100 has two zeros and we put two zeros after 47 to get the product.

Do you see the pattern? If we multiplied 47 times 10,000, which has four zeros, we would put four zeros after 47 to get the product 470,000.

Exercise 1.93:

Multiply: (a) 54 • 10 (b) 54 • 100.

Exercise 1.94:

Multiply: (a) 75 • 10 (b) 75 • 100.

Exercise 1.95:

Multiply: 265(483).

Exercise 1.96:

Multiply: 823(794).

Example 1.49:

Multiply: 896(201).

##### Solution

There should be 3 partial products. The second partial product will be the result of multiplying 896 by 0.

Notice that the second partial product of all zeros doesn’t really affect the result. We can place a zero as a placeholder in the tens place and then proceed directly to multiplying by the 2 in the hundreds place, as shown.

Multiply by 10, but insert only one zero as a placeholder in the tens place. Multiply by 200, putting the 2 from the 12. 2 • 6 = 12 in the hundreds place.

Exercise 1.97:

Multiply: (718)509.

Exercise 1.98:

Multiply: (627)804.

When there are three or more factors, we multiply the first two and then multiply their product by the next factor. For example:

to multiply | 8 • 3 • 2 |

first multiply 8 • 3 | 24 • 2 |

then multiply 24 • 2 | 48 |