English version

mental arithmetic

From Longman Dictionary of Contemporary Englishmental arithmeticˌmental aˈrithmetic noun [uncountable]  HMNthe act of adding numbers together, multiplying them etc in your mind, without writing them down I did a quick bit of mental arithmetic.
Examples from the Corpus
mental arithmeticA quick bit of mental arithmetic was enough to make Chrissy weak at the knees.
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