Multiplying and dividing lengths

In your book, Maths Mela (Chapter 5, Far and Near), a shirt for a 10-year-old child needs 1 m 80 cm of cloth. How much cloth is needed for 20 such shirts? Break the length into metres and centimetres and multiply each: 20 × 1 m = 20 m and 20 × 80 cm = 1,600 cm = 16 m. So 20 m + 16 m = 36 m of cloth.

Dividing works the same way. A road 12 km 600 m long is laid in 6 days, with the same length each day. 12 km ÷ 6 = 2 km, and 600 m ÷ 6 = 100 m. So 2 km 100 m of road is laid each day.

A shop sells cloth for bags at ₹100 for 5 m. Then 1 m costs ₹100 ÷ 5 = ₹20. A double number line helps: one line shows metres and the other shows rupees. 5 m costs ₹100, so 10 m (double) costs ₹200, 20 m costs ₹400 and 40 m costs ₹800. And ₹2,000 is 20 times ₹100, so it buys 20 × 5 m = 100 m.

Anita is embroidering the border of a sari. She needs 1 m of thread for 50 cm of border. A 5 m border is 10 lots of 50 cm, so she needs 10 × 1 m = 10 m of thread. At ₹50 for 1 m of thread, that costs 10 × ₹50 = ₹500.