HSK 7-9
不厌其烦
búyàn-qífánnot to mind taking all the trouble (idiom); to take great pains; to be very patient
老师不厌其烦地重复解释。
Lǎo shī bú yàn qí fán dì chóng fù jiě shì.
The teacher patiently explained it over and over.
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Appears across levels (7-9) in the HSK 3.0 word list.