Using Mental Math to Multiply

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

1. 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:

1. What value makes the equation below true? (Analyze) 
   3 x ? = 90 
2. A bicycle costs $210. How much will be paid for 20 such bicycles?
3. A carton holds 20 packets of biscuits. Each packet has 10 biscuits. How many biscuits can be packed in 30 packets?
4. 4 folders each have 50 sheets of paper inside them. How many sheets of paper are there altogether?
5. 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: