Fractions and Decimals Blog 8

Fractions and Decimals

Day 8

Hello Learner,

I hope you all have completed the previous work given in earlier blogs. In the previous blog, you have learned about expanded form of decimals, comparison of decimals.

Today, we will be learning more about multiplication of decimal numbers.

At the end of today's topic, you will be able to 
(i) multiply decimal numbers.
(ii) solve problems related to it.

So, let begin the topic.......

                             MULTIPLICATIONS OF DECIMALS

How to Multiply Decimals

Just follow these steps:

• Multiply normally, ignoring the decimal points.
• Then put the decimal point in the answer - it will have as many decimal places as the two original numbers combined.

In other words, just count up how many numbers are after the decimal point in both numbers you are multiplying, then the answer should have that many numbers after its decimal point.

How Does It Work?

Because when you multiply without the decimal point, you are really shifting the decimal point to the right to get it out of the way:

Original:		1 Move:		2 Moves:		3 Moves:
0.03 × 1.1		0.3 × 1.1		3. × 1.1		3. × 11.

Then we do the (now easy) multiplication:

3. × 11. = 33.

But remember, we did 3 Moves of the decimal point, so we need to undo that:

3 Moves:		2 Moves:		1 Move:		Correct
33.		3.3		0.33		0.033

Here are some more examples:

Let us all try to solve question given in 'Try These' box page 50.

1. Find (i) 2.7  X  4 

multiply without decimal 27 X 4 =   108
2.7 has one decimal place,
4 has zero decimal place,
so total decimal place is 1
so put the decimal from right counting one=    10.8

(ii) 1.8 X 1.2
multiply without 18 X 12 = 216
1.8 has one decimal place,
1.2 has one decimal place,
so total decimal place is 2
so put the decimal from right counting two= 2.16

Click on the link : https://www.youtube.com/watch?v=7KapMHAK_vI


          Multiplication of Decimal Numbers by 10, 100, 10,000

RULE: When a decimal number is multiplied by 10, 100, 10,000 ...., the digits in the produt are same as in the decimal number but the decimal point in the product is shifted to the right by as, many of places as there are zeros over one.

Click the link : https://www.youtube.com/watch?v=X1HgWTsccKI

10 × 0 . 4 9  = 04.9 = 4.9   Move the decimal point one step to the right (10 has one zero).

100 × 2 . 6 5   = 265. = 265     Move the decimal point two steps to the right (100 has two zeros). The number 265. is 265 (as shown above).

1000 × 0 . 3 7 0 = 3 7 0 . = 370       1000 means we move the point three steps. Write a zero at the end of 0.37 so that the decimal point can “jump over to” that place.

Now, let us try to solve exercise 2.5 of math textbook.

Q1. (i)  0.2 X 6  = 1.2
       (ii) 8 X 4.6 = 36.8

Q2. (i) Area of rectangle = L X B

Q3. (i) 1.3 X 10 = 13
       (ii) 36.8 X 10= 368

Q4. Multiply 55.3 by 10

Q5. (i) 2.5 X 0.3 =  0.75
       (ii) 0.1 X 51.7 = 5.17

Home Assignment
1. Complete Ex 2.6 Pg 52 in your maths note book.