The word 'two-fifths' is a modern English compound formed by directly combining the cardinal number 'two' with the plural ordinal 'fifths'. 'Two' originates from the Proto-Germanic 'twai', tracing back to the Proto-Indo-European 'duwo', a fundamental numeral. 'Fifth' derives from 'five', from Proto-Germanic 'fimf', itself from PIE 'pénkʷe'. The suffix '-th' is a native English pattern for forming ordinals. The compound's logic is purely arithmetic, representing a fraction where the first element counts the parts and the second element names the size of each part, a transparent and analytical method of word formation in English.

Structure: two (2) + fifths (plural of fifth) - two: From Old English 'twā', meaning the number 2. It functions as the numerator in the fraction. - fifths: From Old English 'fīfta' (fifth), derived from 'fīf' (five) + the ordinal suffix '-th'. The '-s' indicates the plural form, functioning as the denominator representing one of five equal parts.