Sonu's screws

Sonu's mother was fixing a toy with a screw, but the two pieces would not join: this screw is not the right size, she said. She took another screw from the box and the toy was fixed. To Sonu the two screws looked the same. Looking closely, he saw they were slightly different lengths. Such a tiny difference, and it mattered!

This happens a lot. A carpenter measuring a shelf space of 2.9 feet who just says 3 feet will make a shelf that doesn't fit. A tailor who needs 2.7 m of cloth can't make do with 2 m. We need numbers between the whole numbers.

Put a screw against a ruler. Which ruler helps you measure accurately? The one with finer divisions, because the difference Sonu wants is smaller than a whole centimetre.

On a ruler, the space between two numbers is divided into 10 equal parts. A length of 2 and 7/10 cm means: go from 0 to 2, then take 7 of those tenth-parts. We read it two and seven-tenth centimetres. In the same way, 3 and 2/10 cm is three and two-tenth centimetres.

Careful: 2 and 7/10 (written 2 7/10) is not 2 × 7/10. It means 2 wholes and 7 tenths.

Why split the unit at all? Because whole centimetres were too rough to tell the two screws apart. When we need exact measures, we use smaller units. That is what this whole chapter is about.