lesson
An approximation is not exact
1 · Learn
Some measurements use imperial units. Useful approximate comparisons include 1 inch about 2.5 cm, 1 pound about 0.45 kg and 1 imperial pint about 0.57 litres.
State that these are approximations. Different pint systems exist, so identify the unit used. Do not use a rough classroom conversion where an exact or safety-critical value is needed.
2 · Worked example
Using 2.5 cm per inch, I estimate 8 inches as 20 cm. The approximation helps visualise a length; it is not an exact definition.
3 · Your turn
Using the supplied approximations, estimate 4 inches in centimetres and 2 pounds in kilograms.
Check your answer
About 10 cm and about 0.9 kg.
4 · Apply your learning
Compare labelled measuring tools with an adult and distinguish exact metric relationships from approximate imperial comparisons.
Adult guidance and safety
Model the calculation or measurement before relying on a procedure. Explain estimates, units and checks. Use a ruler/protractor and suitable diagrams where needed; text examples alone do not establish practical skill or fluency.
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Check a calculation · Year 5 onward
Use this only when your teacher or adult allows a calculator for the task. Show your thinking first. It does not replace mental or written arithmetic and does not mark your lesson answer.
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