to suddenly understand

茅塞顿开

(máo sè dùn kāi)

A: I couldn't figure out the answer to this question for a while. But once you pointed it out, I suddenly understood. Thanks a lot. 

这道题目,我本来想不出来。 听了你的话,我突然茅塞顿开。真的谢谢你。

(zhè dào tímù,wǒ běnlái xiǎnɡ bù chūlái, tīnɡle nǐ de huà,wǒ tūrán máo sè dùn kāi, zhēnde xièxiè nǐ.)

B: You're welcome. 

不客气。

(bù kèqì.)