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Pupils compare two rules for converting Celsius to Fahrenheit, one accurate and one approximate.

Practical details

Suitability: National Curriculum levels 7 to 8
Time: 20 to 40 minutes
Resources: Pencil, calculator and paper; rulers and graph paper available but provided only on request

Representing: Select a way of comparing the two methods, for example, using a table or graph.
Analysing: Explore the effect of varying the temperature; make accurate calculations or graphs, recording methods systematically. Deduce when the approximate method gives an answer that is too high.
Interpreting and evaluating: Interpret their tables and graphs to solve the problem, relating their findings to the original context.
Communicating and reflecting: Communicate their reasoning and findings clearly.

Check that pupils understand the context, for example you could show examples of key temperatures e.g. body temperature and temperature of freezer.

Americans do not use metric units for temperatures, they use degrees Fahrenheit; in Europe, we measure temperature in Celsius or Centigrade.
John and Anne have two different ways of converting from degrees Centigrade to degrees Fahrenheit.
One says their method is accurate and the other says their method is near enough for most purposes.

Pupils can tackle this task in different ways, but they might be expected to:

use algebraic and graphical methods to solve simultaneous linear equations in two variables.