Multiply Simple Decimal Numbers by Whole Numbers

In this activity, we will be multiplying decimal numbers by whole numbers.

 

The first stage is to adjust the question so that there are no decimal numbers.

The second stage is to carry out the multiplication of the whole numbers.

In the third stage, we adjust the answer back to how it should be with decimals.

 

Example 1

Multiply 7.8 x 7

 

Answer

Stage 1

Adjust 7.8 so it becomes a whole number.

We do this by multiplying it by 10 to get rid of the decimal place.

7.8 × 10 = 78

This will mean that our answer will now be 10 times too big. But that is fine!

 

Stage 2

Now we work out 78 × 7.

 

Stage 3

Now, remember that this is 10 times too big.

So we divide 546 ÷ 10 = 54.6.

 

7.8 x 7 = 54.6

 

The key to multiplying with decimals is to count the number of decimal places (numbers after the decimal point). There should always be the same number of decimal places in your answer as in the original calculation.

 

In the question above, there was one number after the decimal point in the question (8), so the answer needed to have one decimal place too. (6)

 

 

Example 2

Multiply 3.68 x 5

 

Answer

Stage 1

Adjust 3.68 so it becomes a whole number.

We do this by multiplying it by 100 to get rid of the two decimal places.

3.68 × 100 = 368

This will mean that our answer will now be 100 times too big. But that is fine!

 

Stage 2

Now we work out 368 × 5.

 

Stage 3

Now, remember that this is 100 times too big.

So we divide 1840 ÷ 100 = 18.40 

 

Remember that there were two decimal places in the question  (68), so there must be two in the answer (40)

 

3.68 x 5 = 18.40

 

However, because the final number after the decimal point is a zero, we can remove it to get the answer 18.4

 

Let's try some questions now!