The numbers on blue shapes are the sums of the two hidden numbers on red circles at the ends of the lines. What are the hidden numbers?
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Topics: Starter  Arithmetic  Problem Solving  Puzzles  Simultaneous Equations
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How difficult would the Pentadd puzzle be without the diagram? Imagine five numbers, randomly chosen, are added together in pairs to produce the following ten sums in no particular order. Can you figure out what those five numbers were?
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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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