lesson
A letter can stand for a number
1 · Learn
A formula expresses a relationship. If a rectangle has length l and width w, its perimeter is 2l + 2w. Substitution replaces letters with values while preserving the operations and units.
The same letter represents the same value within a calculation. Writing 2l means 2 multiplied by l, not a two-digit number ending in l.
2 · Worked example
For l = 9 cm and w = 4 cm, P = 18 + 8 = 26 cm. This measures the boundary, not the enclosed area.
3 · Your turn
Use P = 2l + 2w for l = 12 cm and w = 5 cm.
Check your answer
P = 24 + 10 = 34 cm.
4 · Apply your learning
Invent a simple cost rule with no hidden charges and test it using two different input values.
Adult guidance and safety
Ask the learner to explain the method and check it using a different representation. Supply place-value grids, fraction strips, accurate diagrams and measuring tools where needed. These examples need varied practice; a short correct answer alone does not establish 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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