Measuring fairly
Two friends measure one bench in hand-spans. A hand-span is thumb tip to little finger tip. One counts 6. The other counts 8.
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The idea
The bench did not change. The hands did. A big hand covers more bench, so it needs fewer spans.
A unit is what we count in. A hand-span is an informal unit. A hand-span changes from person to person.
A standard unit is the same for everyone. The centimetre (cm) is one.
| Measured with | Hand size | Count |
|---|---|---|
| Big hand-span | 20 cm | 6 spans |
| Medium hand-span | 15 cm | 8 spans |
| Ruler | — | 120 cm |
120 cm ÷ 20 cm = 6 spans. Also 120 cm ÷ 15 cm = 8 spans. The ruler gives 120 cm to everyone.