put two and two together idiom informal
根据现有情况推论;综合判断
to guess the truth about a situation from what you have seen or heard
"How did you know they were having an affair?" "I'd seen them out together a couple of times, so I just put two and two together." “你怎么知道他们有婚外情？”“我好几次见到他们一起外出，所以就这么推断。”