# Multiply Decimals

In this worksheet, students will calculate the product when multiplying decimals by both whole numbers or another decimal. Students will practise taking out the decimal point and then re-inserting it after they have calculated the multiplication sum.

Key stage:  KS 4

GCSE Subjects:   Maths

GCSE Boards:   Pearson Edexcel, OCR, Eduqas, AQA

Curriculum topic:   Number, Fractions, Decimals and Percentages

Curriculum subtopic:   Structure and Calculation, Decimal Fractions

Difficulty level:

### QUESTION 1 of 10

Multiplying decimals is relatively easy if you follow these 3 simple steps:

1) Take out the decimal points;

2) Calculate the multiplication sum as usual with whole numbers;

3) Put the decimal points back in.

e.g. 8.6 x 4

Step 1: Take out the decimal point, so we now have 86 x 4

Step 2: Complete the multiplication sum as usual, 86 x 4 = 344

Step 3: If you look at the original question, we had one number after the decimal point (8.6 x 4) so we must have one number after the decimal point in the answer, so 8.6 x 4 = 34.4

e.g. 3.2 x 5.6

Step 1: Take out the decimal point, so we now have 32 x 56

Step 2: Complete the multiplication sum as usual, 32 x 56 = 1792

Step 3: If you look at the original question, we had two numbers after the decimal point (3.2 x 5.6) so we must have two numbers after the decimal point in the answer, so 3.2 x 5.6 = 17.92

In this activity, you will need to calculate the product when multiplying decimals by both whole numbers or another decimal.

You should follow the three steps described above, taking out the decimal point and then re-inserting it after you have calculated the multiplication sum as usual.

How many numbers will appear after the decimal point in the answer to the sum below?

3.7 x 8

0

1

2

Use the three steps to calculate:

9.1 x 2

Consider the calculation:

3.74 x 6

Then complete the sentence below.

Use the three steps to calculate:

2.5 x 4

Consider the calculation:

8.31 x 15

Then complete the sentence below.

Consider the calculation below:

3.2 x 1.8

Imagine that a student has written the two lines of working below.

One of the lines in this working has an error.

Choose if each line is correct or incorrect.

Consider the calculation:

7.5 x 2.9

Then complete the sentence below.

Use the three steps to calculate:

8.3 x 2.4

How many numbers will be after the decimal point in the answer to the question below?

9.12 x 3.6

1

2

3

Use the three steps to calculate:

4.15 x 2.83

• Question 1

How many numbers will appear after the decimal point in the answer to the sum below?

3.7 x 8

1
EDDIE SAYS
The original question has one decimal in it, and this decimal has one number after the decimal point. Our answer will have the same number of decimal places, which is 1.
• Question 2

Use the three steps to calculate:

9.1 x 2

18.2
EDDIE SAYS
Taking out the decimal point gives us the sum: 91 x 2 = 182 The original question has one decimal, which has one number after the decimal place so, again, our answer must have the same. If we add the decimal point back in one place from the end of the number, we are left with 18.2 Great focus, hopefully you're getting the hang of this!
• Question 3

Consider the calculation:

3.74 x 6

Then complete the sentence below.

EDDIE SAYS
Did you remember that the first step of the process is to remove the decimal points? Taking out the decimal point gives us: 374 x 6 = 2244 Our original question had one decimal with two decimal places in it. This means that our answer must have the same. If we add the two decimal places back in, our final answer becomes: 22.44
• Question 4

Use the three steps to calculate:

2.5 x 4

10
10.0
EDDIE SAYS
Let's follow our steps: 1) Remove the decimal point to make: 25 x 4 2) Complete the multiplication as usual. Do we need to use a grid for this? A neat little trick for multiplying by 4 is to double it and then double it again. If we do this, we reach 100 3) Our original question had one decimal with one decimal point in, so our answer should have the same. If we add the decimal point back in, one space from the end, we reach our answer: 10.0 When we have a 0 after the point in the tenths column, we can simply write the whole number as an answer, which is 10
• Question 5

Consider the calculation:

8.31 x 15

Then complete the sentence below.

EDDIE SAYS
The original question has one decimal with two numbers after the decimal places, so the answer must have the same. 831 x 15 = 12465 When we add back in the two decimal places in, the final answer becomes 124.65 Did you get that?
• Question 6

Consider the calculation below:

3.2 x 1.8

Imagine that a student has written the two lines of working below.

One of the lines in this working has an error.

Choose if each line is correct or incorrect.

EDDIE SAYS
We had two decimals in our question here - did you spot that? Do we need to change our approach at all? No! Taking out the decimal points gives us: 32 x 18 = 576 Which makes the student's first line of working correct. The original question has two decimals, both with one decimal place. So we need to have 1 + 1 = 2 decimal places in our answer. If we add the two decimal places back into 576, we reach an answer of 5.76 As the student wrote an answer of 57.6, this line of their working is incorrect. How did you find being the examiner there?
• Question 7

Consider the calculation:

7.5 x 2.9

Then complete the sentence below.

EDDIE SAYS
The original question has two decimals with one number after each of the decimal places, so the answer must have the same. 1 + 1 = 2 decimal places in the answer. 75 x 29 = 2175 When we add back in the two decimal places in, the final answer becomes 21.75
• Question 8

Use the three steps to calculate:

8.3 x 2.4

19.92
EDDIE SAYS
Taking out the decimal points gives us: 93 x 24 = 1992 The original question has two decimals, each with one the decimal place, so the answer must have the same. 1 + 1 = 2 decimal places If we add two decimal places back into 1992, we reach the answer of 19.92
• Question 9

How many numbers will be after the decimal point in the answer to the question below?

9.12 x 3.6

3
EDDIE SAYS
The original question has two decimals, one with one decimal places and the other with two. Our answer needs to have the same number of decimal places. 1 + 2 = 3 decimal places in our answer
• Question 10

Use the three steps to calculate:

4.15 x 2.83

11.7445
EDDIE SAYS
This one seems trickier but it's just slightly longer; we can still follow our three steps as usual. Taking out the decimal points gives us: 415 x 283 = 117445 The original question has two decimals, each with two decimal places, so our answer will have four numbers after the decimal place. If we add these back into 117445, we reach an answer of: 11.7445 Great work completing that activity! Hopefully you are feeling much more confident when multiplying decimals now and following our simple three steps.
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