Add related fractions

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. Mei eats one half of a pandan roll cake. Kavin eats three eighths of the same pandan roll cake. What fraction of the pandan roll cake do they eat altogether? Here is the pandan roll cake as one whole bar. Mei eats one half, so we cut the bar into 2 equal parts. Shade 1 part for Mei. Kavin eats three eighths. But the bar has only 2 equal parts, so we cannot show three eighths yet. We need 8 equal parts. Cut each of the 2 parts into 4 equal parts. 2 times 4 is 8. Mei's shaded part now has 4 small parts. 1 times 4 is 4. Mei's part is now 4 of the 8 parts. Kavin eats 3 parts. Now we add four eighths and three eighths. We count the shaded parts: 4 plus 3 is 7. The bar still has 8 equal parts, so the denominator stays 8. Together, they eat seven eighths of the pandan roll cake. We subtract in the same way, after making the parts the same size.

Lin has seven eighths of a chocolate bar. She gives one quarter of the chocolate bar to her brother. What fraction of the chocolate bar does she have left? Try it first. Pause if you like. Here is the chocolate bar as one whole. It has 8 equal parts, and Lin has 7 of them. She gives away one quarter. We change one quarter to a fraction with denominator 8. 4 times 2 is 8, and 1 times 2 is 2. So she gives away 2 of the 8 parts. Now, seven eighths minus two eighths. 7 minus 2 is 5, and the denominator stays 8. Lin has five eighths of the chocolate bar left.

Bala paints one fifth of a fence. Siti paints three tenths of the same fence. What fraction of the fence do they paint altogether? Give your answer in its simplest form. Try it first. Pause if you like. Here is the fence as one whole, with 5 equal parts. Bala paints 1 part. Siti paints three tenths, so we need 10 equal parts. Cut each of the 5 parts into 2 equal parts. 5 times 2 is 10, and 1 times 2 is 2. Siti paints 3 parts. Two tenths plus three tenths is five tenths. The question asks for the simplest form. 5 and 10 can both be divided exactly by 5. 5 divided by 5 is 1, and 10 divided by 5 is 2. Together, they paint one half of the fence. First make the parts the same size. Then add or subtract the numerators, and keep the denominator.