Adding and subtracting weights

In your book, Maths Mela (Chapter 8), Rathna goes to the grocery store. She buys 2 kg 500 g of rice for daily use and 1 kg 750 g more for the Pongal festival. How much rice does she buy in all?

We add like units: kilograms with kilograms, grams with grams. 2 kg + 1 kg = 3 kg. 500 g + 750 g = 1,250 g. But 1,000 g make 1 kg, so 1,250 g = 1 kg 250 g. Put the extra 1 kg with the kilograms: 3 kg + 1 kg 250 g = 4 kg 250 g. Another way is to change both into grams: 2,500 g + 1,750 g = 4,250 g = 4 kg 250 g.

How much more rice did she buy for home than for Pongal? We need 2 kg 500 g − 1 kg 750 g. We cannot take 750 g from 500 g, so we change 1 kg into 1,000 g: 2 kg 500 g becomes 1 kg 1,500 g. Now 1,500 g − 750 g = 750 g and 1 kg − 1 kg = 0 kg. So she bought 750 g more for home. In grams: 2,500 g − 1,750 g = 750 g.

The book gives many problems like this. Aarav is helping his grandfather at the fruit stall and lifts two baskets of apples. In a Gurdwara's community kitchen, 65 kg of flour is bought, 42 kg 275 g is used for langar, and 52 kg 500 g more is bought the next day. First subtract: 65 kg − 42 kg 275 g = 22 kg 725 g. Then add: 22 kg 725 g + 52 kg 500 g = 75 kg 225 g of flour in the store.

Remember: when the grams add up to 1,000 g or more, carry 1 kg. When there are not enough grams to take away, change 1 kg into 1,000 g.