### Key Concepts

- Understand the block multiplication of multiples of 10

Various examples of mental math problems

# Introduction

**What do you mean mental maths?**

Mental math involves simplifying equations to know the exact or estimated answer. Generally, multiplication problems are solved using mental math by reducing the number by 10s and 100s to solve the larger numbers.

**Example 1:**

A regular box of pencils has 20 pencils. A jumbo box holds 40 pencils. There are 4 boxes of regular pencils on one shelf and 4 boxes of jumbo pencils on another shelf. How many pencils are on each shelf?

**Solution:**

The number of pencils in a regular box is = 20 pencils

- The number of boxes of regular pencils on one shelf is = 4 boxes

Think about the tens

Multiply 4 by the number of tens.

4 x 20 = 4 x 2 tens

4 x 20 = 8 tens

4 x 20 = 80

The number of pencils on one shelf is = 80.

2. The number of pencils in the jumbo box = 40 pencils

The number of boxes of jumbo pencils on another shelf = 4 Boxes

Sometimes the basic multiplication fact makes the product look different.

4 x 40 = 4 x 4 tens

4 x 40 = 16 tens

4 x 40 = 160 tens

The number of pencils on another shelf = 160.

**Example 2:**

A regular box of fruits has 40 fruits. A jumbo box holds 80 fruits. There are 5 boxes of regular fruits on one shelf and 5 boxes of jumbo fruit on another shelf. How many fruits are on each shelf?

**Solution:**

The number of fruits in a regular box = 40 fruits

The number of boxes of regular fruits on oneself = 5 boxes

Think about this thing, multiply 5 by the number of tens

5 x 40 = 5 x 4 tens

5 x 40 = 20 tens

5 x 40 = 200

The number of fruits on one shelf = 200.=

3. The number of fruits containing jumbo box = 80 fruits

Sometimes the basic multiplication fact makes the product look different.

5 x 80 = 5 x 8 tens

5 x 80 = 40 tens

5 x 80 = 400

The number of fruits on another shelf = 400.

**Example 3:**

Find the number of ten’s in 6 x 70.

Solution:

Given 6 x 70

®Check the place value of 70.

In ten’s place, there are “7 tens”.

The number of ten’s in 6 x 70 = 420

There are 20 tens in 6 x 70.

**Example 4:**

Find the Product of 9 x 80.

**Solution:**

Given 9 x 80

9 x 80 = 9 x 8 tens

9 x 80 = 72 tens

9 x 80 = 720

The product of 9 x 80 = 720.

**Example 5:**

Find the missing number of the equation that makes it true.

9 x? = 630

Solution:

Given 9 x? = 630

9 x? = 63 tens

From the multiplication of table, we know that 9 x 7 = 63.

If we multiply by 10 then 0 is added at the one’s place, i.e., 10 x 7 = 70

The missing number of equation that makes it true = 70.

**Example 6:**

Alex and his family are going to the movie theater. There are 2 adults and 3 children. How much will the tickets cost?

**Solution:**

The cost of the movie ticket for child = $10

Total cost for the movie ticket of 3 children = 3 x $ 10 = $30

The cost of the movie ticket for adult = $30

Total cost for the movie ticket of 2 adults = 2 x $30 = $60

The ticket costs for both adults and children = $30 + $60 = $90

**Assessment**

- Find the product of 2 x 30 by using block multiplication.
- Find the missing value of 8 x ___ = 640.

## Exercise:

- What value makes the equation below true? (Analyze)

3 x ? = 90 - A bicycle costs $210. How much will be paid for 20 such bicycles?
- A carton holds 20 packets of biscuits. Each packet has 10 biscuits. How many biscuits can be packed in 30 packets?
- 4 folders each have 50 sheets of paper inside them. How many sheets of paper are there altogether?
- What value makes the equation below true?

10 x ? = 80

### What we have learned:

- Block multiplication of multiples of 10

#### Let us Summarize

#### Concept Map:

#### Related topics

#### Addition and Multiplication Using Counters & Bar-Diagrams

Introduction: We can find the solution to the word problem by solving it. Here, in this topic, we can use 3 methods to find the solution. 1. Add using counters 2. Use factors to get the product 3. Write equations to find the unknown. Addition Equation: 8+8+8 =? Multiplication equation: 3×8=? Example 1: Andrew has […]

Read More >>#### Dilation: Definitions, Characteristics, and Similarities

Understanding Dilation A dilation is a transformation that produces an image that is of the same shape and different sizes. Dilation that creates a larger image is called enlargement. Describing Dilation Dilation of Scale Factor 2 The following figure undergoes a dilation with a scale factor of 2 giving an image A’ (2, 4), B’ […]

Read More >>#### How to Write and Interpret Numerical Expressions?

Write numerical expressions What is the Meaning of Numerical Expression? A numerical expression is a combination of numbers and integers using basic operations such as addition, subtraction, multiplication, or division. The word PEMDAS stands for: P → Parentheses E → Exponents M → Multiplication D → Division A → Addition S → Subtraction Some examples […]

Read More >>#### System of Linear Inequalities and Equations

Introduction: Systems of Linear Inequalities: A system of linear inequalities is a set of two or more linear inequalities in the same variables. The following example illustrates this, y < x + 2…………..Inequality 1 y ≥ 2x − 1…………Inequality 2 Solution of a System of Linear Inequalities: A solution of a system of linear inequalities […]

Read More >>
Comments: