の内(うち)でOf the, Among(st)

～のうち(で) and のなかで are similar expressions that both mean 'among', but their usage can differ. ～のうち(で) is most often used with number + counter phrases. For example, 5人のうち4人が合格した ('Out of 5 people, 4 passed.'). With non-numerical groupings, like スポーツの中でどれが一番好き？ ('Among sports, which do you like best?'), のなかで is more common, especially in speech.

Both ～のうち(で) and のなかで express the idea of 'among' or 'out of,' used when selecting or describing something from a group. のうち is often (though not exclusively) used with number + counter phrases, as in 10人のうち5人が車を持っている ('Out of 10 people, 5 own a car'). のなかで is more common and casual in speech, while のうち has a slightly more formal tone.

Both ～のうち(で) and ～は～の一つだ are used to identify things within a group or category. ～のうち(で) is most often paired with number + counter phrases to mean 'among', for example, ５人のうち４人が合格した ('Out of 5 people, 4 passed.'). In contrast, ～は～の一つだ identifies something as part of a group, e.g., バナナは果物の一つだ ('A banana is one type of fruit.').

Both ～のうち(で) and 同士 describe relationships within a group, but differ in nuance. ～のうち(で) indicates selection from a group, e.g. ３人のうちで誰が一番早い？ ('Who is the fastest among the three?'). 同士 emphasizes mutual relationships or shared characteristics, e.g. 学生同士で助け合う ('Help each other as students').