Explanation: The abacus was the calculator of accountants in the past, a good accountant had to be good at using the abacus, therefore, “an iron abacus” was a compliment for a good accountant. In modern times the abacus has been replaced by the calculator, this term is still in use to describe a person who is shrewd , or petty and scheming..

Ex.1: Mrs. Li is really a shrewd lady. She can cook one jin of ribs into various dishes and feed the whole family with meat for a couple of days.

Ex.2: I advise you to think it over, don't miscalculate.

Explanation: In ancient times, when officials finished work and left the office, there will be a rite of drum beating. Now this expression indicates running into trouble and backing out of plan or changing one's idea..

Ex: After spending six months preparing for the TOEFL test, Xiao Zhou gave up at the last moment.