About four-fifths of the students in my class are girls.

• About four-fifths of the students in my class are girls. (我班上的學生約有五分之四是女生) (正)
• About four-fifth of the students in my class are girls. (誤)
• About four-fives of the students in my class are girls. (誤)

解說：表示幾分之幾時，分子用基數詞 (cardinal)，分母用序數詞 (ordinal)，若分子大於一，則分母的序數詞後面要加 s。