Decimals
UK Primary (3-6)
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Convert from fractions to decimals
Lesson

Dashes to dots

We've already looked at how to change decimals to fractions. Now let's look at how to go the other way and change fractions to decimals.

Think about the names of the columns in our place value table: tenths, hundredths, thousandths and so on. So to change fractions to decimals, it's easiest if we have a denominator of $10,100,1000$10,100,1000 and so on (so it matches the numbers in the place value table).

Examples

question 1

Convert $\frac{3}{10}$310 to a decimal.

This fraction means "three tenths," so to write it in the place value table, it would be

 

Let's look at another example.

question 2

Convert $\frac{47}{100}$47100 to a decimal.

This time, we need to make sure our decimal finishes in the hundredths column, so we would write this as $0.47$0.47

 

What about when we have a denominator that is not a power of 10?

If we have a fraction with a denominator that is not a power of $10$10, the we want to convert it so it does.

So the process goes:

1) Find a number you can multiply the denominator by to make it a power of $10$10.
2) Multiply the numerator and denominator by that number.
3) Then write down just the top number, putting the decimal point in the correct spot.

 

Examples

question 3

Question: Convert $\frac{4}{5}$45 to a decimal.

Think: To change the denominator to $10$10, we need to know how many tenths are in $1$1 fifth.  

Do: From our work with fractions, we know that $2$2 tenths = $1$1 fifth, so for every fifth we have (and we have $4$4 of them) that is $2$2 tenths.  So $4$4 fifths, is the same as $8$8 tenths.  

 

Let's look at another example.

Question 4

Question: Convert $\frac{159}{300}$159300 to a decimal.

Think: To change the denominator $100$100, we ask ourselves, how many three-hundredths are in a hundredth. The answer is $3$3.  So for every $3$3 three-hundredths we have (and we have $159$159 of them) we have $1$1 hundredth.  This means that $159$159 three-hundredths is the same as $53$53 hundredths.  What we are doing here is dividing $300$300 by $3$3. Whatever we do to the denominator, we have to do to the numerator.

Do$\frac{159}{300}$159300 = $\frac{53}{100}=0.53$53100=0.53

 

Converting mixed numbers

We use the same process to convert mixed numbers.

Example

question 5

Question: Convert $4\frac{567}{1000}$45671000 to a decimal.

Think: The $4$4 belongs in the units column, then the $567$567 will come after the decimal point.

Do: $4.567$4.567

 

Writing decimals

Examples

Question 6

QuestionWrite $15+\frac{4}{10}+\frac{5}{1000}$15+410+51000

Think: Where these numbers belong in the place value table (look- we have no hundredths).

Do$15.405$15.405

 

We may need to do some conversion or even addition before we write a decimal.

Question 7

QuestionWrite $4+\frac{4}{5}+\frac{15}{100}$4+45+15100

Think: This would be $4+\frac{8}{10}+\frac{15}{100}$4+810+15100. In $\frac{15}{100}$15100, the one belongs in the tenths column, so we will have to add this on to our other tenths. In other words, we could think of it as $4+\frac{8}{10}+\frac{1}{10}+\frac{5}{100}$4+810+110+5100

Do: $4.95$4.95

More Worked Examples

Question 8

Write the fraction $\frac{1}{2}$12 as a decimal.

Question 9

Write the fraction $\frac{349}{500}$349500 as a decimal.

Question 10

Evaluate $8+\frac{8}{10}+\frac{1}{100}+\frac{1}{1000}$8+810+1100+11000, expressing your answer as a decimal.

Question 11

Evaluate $\frac{3}{10}+\frac{3}{50}$310+350, expressing your answer as a decimal.

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