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hard nut to crack▼(challenging problem to solve)noun[UK: hɑːd nʌt tuː kræk] [US: ˈhɑːrd ˈnət ˈtuː ˈkræk]
tough nut to crack▼(challenging problem to solve)noun[UK: tʌf nʌt tuː kræk] [US: ˈtəf ˈnət ˈtuː ˈkræk]
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