# Questions tagged [mental-arithmetic]

Mental arithmetic comprises arithmetical calculations using only the human brain, with no help from calculators, computers, or pen and paper.

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### Expected Length of Walk on Truncated Icosahedron

Consider a truncated icosahedron with 12 pentagons and 20 hexagons. Starting from a hexagonal face, we go to any neighboring polygon randomly with equal probability. What is the expected number of ...
• 151
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### In time t, a dog does 3 jumps whereas a fox does 5. If distance covered in a jump by a dog is thrice that of the fox, find ratio of speed of dog : fox

This looks (and I think it is) a pretty simple problem. My reasoning is as follows : ...
• 361
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### Sum of values from 1 to 100 excluding values with digits of 7 and/or 8

I want to quickly sum values from 1 to 100, but exclude values with digits of 7 and/or 8 (e.g., 7,8,17,18,70,78,....) from the sum. This is a mental math problem that I want to do in a really quick ...
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### Is there a reasonably accurate and easy way to approximate lbs-stones or kg-stones (and vice versa)?

For example, when I need to talk with my American friends about body weight: $\mathrm{KG}\times2+10\%$ ($100\times2=200$, $200+10\%=220$... Pretty accurate and easy to do mentally, both ways) However, ...
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