One of the keys on a calculator is faulty. When you press it you get a different number. Some of the results produced by the calculator are shown below. Which is the faulty key?

677
204
624
711
538

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• One strategy would be to jot down the digits 0 to 9 on some scrap paper of the back of the exercise book. Next cross out the digits used in the correct calculations as those buttons could not be faulty… Or could they? For example if 7 was the faulty key and always produced a 5 instead of a 7, the calculation 237 – 117 would still produce the correct answer of 120. Did your pupils come up with a better strategy?

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Previous Day | This starter is for 11 February | Next Day

The faulty key is 7
It produces a 2

Note to teacher: Doing this activity once with a class helps students develop strategies. It is only when they do this activity a second time that they will have the opportunity to practise those strategies. That is when the learning is consolidated. Click the button above to regenerate another version of this starter from random numbers.

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 Teacher, do your students have access to computers?Do they have iPads or Laptops in Lessons? Whether your students each have a TabletPC, a Surface or a Mac, this activity lends itself to eLearning (Engaged Learning).

Transum.org/go/?Start=February11

Here is the URL which will take them to a related student activity.

Transum.org/go/?to=six

If you think faulty calculators still have a use try this.

Transum.org/go/?to=broken

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