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adjectival nouns or quasi-adjectives (keiyodoshi), nouns which may take the genitive case particle 'no', noun (common) (futsuumeishi)
easy, smooth, without a hitch, quick
most frequently used adverbially as 〜に-
この国の経済は近年とんとん拍子に成長しています。
This nation's economy is growing by leaps and bounds in recent years.
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