In this Grade 4 Saxon Math lesson from Chapter 5, students learn two methods for multiplying two-digit numbers by a one-digit number: a mental math strategy using place value decomposition (breaking a number like 21 into 20 + 1) and the standard pencil-and-paper algorithm for multiplying ones and tens separately. Students apply these multiplication skills to find products such as 42 × 3 and solve real-world problems involving area, reinforcing the connection between multiplication and rectangle dimensions.
Section 1
📘 Multiplying Two-Digit Numbers, Part 1
Instead of finding , we will solve this problem by multiplying 21 by 3.
Next, you’ll master a mental math shortcut and a pencil-and-paper method for multiplying two-digit numbers.
Section 2
Mental Math
To multiply a two-digit number by a one-digit number, break the two-digit number into its tens and ones. Multiply each part separately, and then add the results together. For example, to solve , think of 21 as . Multiply and , then add .
To solve , think and . Then add . For , calculate and . The final answer is . Let's try . We have and . So, .
Multiplying big numbers in your head is like being a math ninja! Just split the larger number into its tens and ones. Zap each part with the multiplier, then add the results back together. It's a slick trick to solve problems quickly without needing to write anything down, making you look like a total genius.
Section 3
Pencil and Paper
To multiply with pencil and paper, write the larger number on top and the smaller number below it, aligned to the right. First, multiply the bottom number by the ones digit of the top number. Then, multiply the bottom number by the tens digit of the top number. Combine the results to get the final product.
To multiply : First, multiply . Then, multiply . Combine them to get 124. Calculate : Start with the ones, . Then the tens, . The product is 156. For : Multiply the ones, . Then the tens, . Your final answer is 126.
This is the classic way to multiply, like a trusty recipe for success! Stack the numbers neatly, then multiply from right to left—first the ones, then the tens. This step-by-step method keeps your calculations organized and ensures you nail the correct answer every single time, especially when numbers get bigger and trickier.
Section 4
Area of a rectangle
To find the area of a rectangle, you must multiply its length by its width. The formula is . The result is always expressed in square units, such as square feet (sq. ft) or square meters (sq. m), representing the total space inside the shape's boundaries.
A rectangular garden is 15 feet long and 5 feet wide. Its area is . To find the area of a poster that is 20 inches by 8 inches, multiply the sides: . A rectangular flag measures 11 meters by 7 meters. The total area inside the flag is .
Figuring out the area of a rectangle is like calculating how much wrapping paper you need for a gift. Just multiply the two side lengths! This handy trick tells you the total number of little squares that can fit inside the shape. It’s a super practical way to use multiplication for real-world measurements and projects.
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Section 1
📘 Multiplying Two-Digit Numbers, Part 1
Instead of finding , we will solve this problem by multiplying 21 by 3.
Next, you’ll master a mental math shortcut and a pencil-and-paper method for multiplying two-digit numbers.
Section 2
Mental Math
To multiply a two-digit number by a one-digit number, break the two-digit number into its tens and ones. Multiply each part separately, and then add the results together. For example, to solve , think of 21 as . Multiply and , then add .
To solve , think and . Then add . For , calculate and . The final answer is . Let's try . We have and . So, .
Multiplying big numbers in your head is like being a math ninja! Just split the larger number into its tens and ones. Zap each part with the multiplier, then add the results back together. It's a slick trick to solve problems quickly without needing to write anything down, making you look like a total genius.
Section 3
Pencil and Paper
To multiply with pencil and paper, write the larger number on top and the smaller number below it, aligned to the right. First, multiply the bottom number by the ones digit of the top number. Then, multiply the bottom number by the tens digit of the top number. Combine the results to get the final product.
To multiply : First, multiply . Then, multiply . Combine them to get 124. Calculate : Start with the ones, . Then the tens, . The product is 156. For : Multiply the ones, . Then the tens, . Your final answer is 126.
This is the classic way to multiply, like a trusty recipe for success! Stack the numbers neatly, then multiply from right to left—first the ones, then the tens. This step-by-step method keeps your calculations organized and ensures you nail the correct answer every single time, especially when numbers get bigger and trickier.
Section 4
Area of a rectangle
To find the area of a rectangle, you must multiply its length by its width. The formula is . The result is always expressed in square units, such as square feet (sq. ft) or square meters (sq. m), representing the total space inside the shape's boundaries.
A rectangular garden is 15 feet long and 5 feet wide. Its area is . To find the area of a poster that is 20 inches by 8 inches, multiply the sides: . A rectangular flag measures 11 meters by 7 meters. The total area inside the flag is .
Figuring out the area of a rectangle is like calculating how much wrapping paper you need for a gift. Just multiply the two side lengths! This handy trick tells you the total number of little squares that can fit inside the shape. It’s a super practical way to use multiplication for real-world measurements and projects.
Book overview
Jump across lessons in the current chapter without opening the full course modal.
Continue this chapter