The numeracy tests cover reading graphs and tables, as well as mental arithmetic questions including percentages and fractions.

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"Mental arithmetic is a more complex and challenging task, which more than 20% of people struggle with, " the study said.

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His use of this extra memory space meant that he could circumvent that perennial pitfall of mental arithmetic, losing one's place.

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Wily suspects may try to raise their baseline readings artificially (by doing mental arithmetic, for example), making later fibs more difficult to detect.

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Calculations by scientists, who are forever totting up times but are often bad at mental arithmetic, will also be much easier if they can just move decimal points instead of multiplying and dividing.

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In teaching maths, for instance, both those countries spend more time on basic arithmetic than on deeper mathematical ideas, emphasise mental arithmetic, rely on standard teaching manuals and favour whole-class (as opposed to group) teaching.

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Ms Truss said an over-reliance on calculators meant children missed the rigorous grounding in mental and written arithmetic they needed to progress.

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