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?
677Topics: Starter  Arithmetic  Calculator  Problem Solving
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Previous Day  This starter is for 11 February  Next Day
The faulty key is 7
It produces a 2
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Numbers and the Making of UsI initially heard this book described on the Grammar Girl podcast and immediately went to find out more about it. I now have it on my Christmas present wish list and am looking forward to receiving a copy (hint!). "Caleb Everett provides a fascinating account of the development of human numeracy, from innate abilities to the complexities of agricultural and trading societies, all viewed against the general background of human cultural evolution. He successfully draws together insights from linguistics, cognitive psychology, anthropology, and archaeology in a way that is accessible to the general reader as well as to specialists." more... 
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If you think faulty calculators still have a use try this.