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mental arithmetic
彼女は暗算が得意で複雑な計算も速くできる。
She's good at mental math and can do complex calculations quickly.
暗算で答えを出すのは難しい問題だった。
It was a difficult problem to solve with mental math.
暗算で合計金額を素早く計算した。
I quickly calculated the total amount with mental math.