The idea in brief. Rename related fractions as equal-sized parts before adding or subtracting; keep the denominator and simplify. Change to equal-sized parts before adding or subtracting. Keep the denominator when counting those parts. In the lesson. In Primary 2, you added fractions with the same denominator. One fifth plus two fifths is three fifths. Today, we will add and subtract related fractions, like one half and three eighths. First make the parts the same size. Then add or subtract the numerators, and keep the denominator.
Lesson video, with the working
Add related fractions
4 min 50 s
The film teaches 3 questions in turn. Each question shows first; its working is written as the film explains it.
Mei eats 1/2 of a pandan roll cake. Kavin eats 3/8 of the same pandan roll cake. What fraction of the pandan roll cake do they eat altogether?
1/2 = (1 × 4)/(2 × 4) = 4/8
1/2 + 3/8 = 4/8 + 3/8 = 7/8
Ans: 7/8
Example 1 of 2
Lin has 7/8 of a chocolate bar. She gives 1/4 of the chocolate bar to her brother. What fraction of the chocolate bar does she have left?
1/4 = (1 × 2)/(4 × 2) = 2/8
7/8 − 1/4 = 7/8 − 2/8 = 5/8
Ans: 5/8
Example 2 of 2
Bala paints 1/5 of a fence. Siti paints 3/10 of the same fence. What fraction of the fence do they paint altogether? Give your answer in its simplest form.
1/5 = (1 × 2)/(5 × 2) = 2/10
1/5 + 3/10 = 2/10 + 3/10 = 5/10
5/10 = (5 ÷ 5)/(10 ÷ 5) = 1/2
Ans: 1/2
How to solve it
Draw the whole as a bar and shade the first fraction. One half is 1 of 2 parts.
Make the parts the same size. Cut each part into 4, so one half becomes four eighths.
Add or subtract the numerators and keep the denominator: 4 plus 3 is 7, so seven eighths.
Subtraction works the same way: seven eighths minus two eighths is five eighths.
If the question asks for the simplest form, divide both numbers by the same number: five tenths is one half.
First make the parts the same size. Then add or subtract the numerators, and keep the denominator.
Practice questions with worked solutions
Question 1 is free to try. The other 7 open in the free trial, each with its solution film.
What are the steps for adding and subtracting related fractions in P3?
This page teaches your child to add and subtract two related fractions, such as halves and eighths or fifths and tenths, within one whole.
What your child learns here
This page teaches your child to add and subtract two related fractions, such as halves and eighths or fifths and tenths, within one whole. It follows the P3 outcome on adding and subtracting related fractions with denominators up to 12, and applies it to short word problems.
Spot related fractions: one denominator is a multiple of the other, like 3 and 6, or 4 and 12.
Rename the fraction with the smaller denominator so both fractions count parts of the same size.
Add or subtract the numerators and keep the denominator, staying within one whole.
Give the answer in simplest form when the question asks for it.
Choose addition for an amount altogether and subtraction for what is left or how much more.
Does the answer make sense?
A sum must be larger than each fraction being added, and a difference or a part left must be smaller than the larger fraction it came from. At this level every answer stays within one whole, so a result above 1 tells your child to check that both fractions were renamed into equal-sized parts.
How to practise these questions
The Basic questions practise adding and subtracting fractions with the same denominator, or where one denominator is a multiple of the other. The Intermediate questions add short stories about parts of a sheet, a banner or a strip, where the part left or the difference is found. The Challenge questions combine three fractions or ask for the part left from a whole, with simplest form at the end. When a fraction sum is stuck, view its solution film together and come back to it later from Revision.
Questions parents ask
What are related fractions? Related fractions have denominators where one is a multiple of the other, such as 1/3 and 5/6, or 3/4 and 1/12. Only one of them needs renaming before adding or subtracting. The Primary 3 syllabus covers adding and subtracting two related fractions within one whole, with denominators up to 12.
Does my P3 child need to simplify the answer when adding fractions? When the question asks for the simplest form, yes. The lessons here simplify as the last step, so your child gets used to asking whether the answer can be reduced.
Is a bar model needed for adding fractions? It is not required, but the lesson film draws one to show why the parts must be the same size. A quick bar on scrap paper also helps with word problems where it is not obvious what to add or subtract.
