(let's) say idiom
（用于引出建议或举例）
used to introduce a suggestion or possible example of something
Try and finish the work by, let's say, Friday. 我建议尽量在星期五前完成这项工作。
Say/Let's say (= if we accept) (that) the journey takes three hours, then you'd be there by two o'clock. 假定这段路程需要3小时的话，你们最晚将在两点钟到达。