difficult problem
tough problem
challenge
She handles this difficult subject with a light touch.
They tried to catch her out with a difficult question.

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Open the tool →难题 (nán tí) means “difficult problem, tough problem, challenge”.
难题 is pronounced nán tí in Mandarin Chinese.
“difficult problem” is 难题 (nán tí) in Chinese.
This page has 2 example sentences using 难题, each with pinyin and an English translation.