To have a clear understanding of the situation in one's mind; to know what's what.
对情况和问题有基本的了解，处理事情有一定把握。
冯德英《迎春花》第八章春玲要先同儒春谈一次，摸摸他的底，心中有数，为她去和老东山交锋做准备。”
Even though he didn't show it, he had a clear picture of the project's progress in his mind.
Learn the next connected idiom: 心中有数 (xīn zhōng yǒu shù) → 数不胜数 (shǔ bù shèng shǔ)
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