Perimeter

In your book, Maths Mela (Chapter 11), Preetha and Adrit's grandmother has made a quilt cover from old clothes. Now she wants to sew lace all the way round its border. She has a red lace 40 units long, a green lace 50 units long and a blue lace 25 units long. Which one is long enough?

The length of the border of a shape is called its perimeter. To find it, add the lengths of all the sides. Suppose the quilt is 15 units long and 10 units wide. Going round it, the sides are 15, 10, 15 and 10 units. So the perimeter is 15 + 10 + 15 + 10 = 50 units. The green lace, 50 units, goes exactly round. The red lace (40) and the blue lace (25) are too short.

When all the sides of a shape are equal, we can multiply instead of adding. The book shows a pentagon, a shape with 5 sides, and every side is 4 cm. Its perimeter is 4 + 4 + 4 + 4 + 4 = 5 × 4 = 20 cm.

Always count every side. A rectangle 8 cm long and 5 cm wide has four sides: two of 8 cm and two of 5 cm. Its perimeter is 8 + 5 + 8 + 5 = 26 cm.