hâm-kháu-sǹg含口算

to perform mental arithmetic

to perform mental arithmetic; to do sums in one's head

English Wiktionary, “含口算” (CC BY-SA 4.0) — https://en.wiktionary.org/wiki/%E5%90%AB%E5%8F%A3%E7%AE%97 · confidence: Medium (Wiktionary)