「難題」

(pos:名詞)
<explanation>
yue:好難解決嘅問題（量詞：個／條）
eng:difficulty; tough issue; challenge
<eg>
yue:呢個係數學界嘅世紀難題，好多數學家用咗成世時間都解決唔到。 (nei1 go3 hai6 sou3 hok6 gaai3 ge3 sai3 gei2 naan4 tai4, hou2 do1 sou3 hok6 gaa1 jung6 zo2 seng4 sai3 si4 gaan3 dou1 gaai2 kyut3 m4 dou2.)
eng:This is a century-old problem in mathematics, which many mathematicians spent their whole lives on it but still could not solve it.