noun, noun or participle which takes 'suru', transitive verb
mental arithmetic; mental calculation
Doing arithmetic in one's head without using a calculator, abacus, or writing. Often used with する to mean 'do mental arithmetic'.
彼は暗算が得意だ。
He is good at mental arithmetic.
買い物のとき、暗算で合計を出
When shopping, I calculated the total in my head.