One of the most useful everyday chengyu, fully idiomatic in modern speech and writing. Frequent in business negotiations, legal contexts, pricing discussions, and family disputes. Pairs naturally with 解决方案, 价格, 规则, 条件.
grammar
Two synonymous pairs: 公平 (fair) plus 合理 (reasonable). Chengyu built from two AB patterns where both halves mean almost the same thing are especially common in legal/business register: 合情合理, 名正言顺, etc.
