Mental arithmetic; doing calculations in one's head without paper or a calculator.
彼は暗算で一桁の掛け算をすぐに答えられる。
He can immediately answer single-digit multiplications by mental arithmetic.
母は買い物で暗算をして、合計を素早く出した。
My mother did the math in her head and quickly worked out the total while shopping.
小学校の授業で暗算の練習をした。
We practiced mental arithmetic in elementary school class.