Jisho

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Noun, Suru verb, Transitive verb
1. mental arithmetic
Wikipedia definition
2. Mental calculationMental calculation comprises arithmetical calculations using only the human brain, with no help from calculators, computers, or pen and paper. People use mental calculation when computing tools are not available, when it is faster than other means of calculation (for example, conventional methods as taught in educational institutions), or in a competition context. Mental calculation often involves the use of specific techniques devised for specific types of problems.
Read “Mental calculation” on English Wikipedia
Read “暗算” on Japanese Wikipedia
Read “Mental calculation” on DBpedia

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