Noun, Noun which may take the genitive case particle 'no'
as likely as not; fifty-fifty; even match; tie
Yojijukugo (four character compound)
鯨が今世紀の終わりに死に絶える可能性は五分五分である。As likely as not, whales will be extinct by the end of this century.
Also written ５分５分【ごぶごぶ】