1630s, "reason, rationale," from Latin ratio "reckoning, numbering, calculation; business affair, procedure," also "reason, reasoning, judgment, understanding," from rat-, past participle stem of reri "to reckon, calculate," also "think" (see reason (n.)). Mathematical sense "relationship between two numbers" is attested from 1650s.
例文
1. In 1978 there were 884 students at a lecturer/student ratio of 1：15.
1978年に884人の学生がいて、教師と学生の比は1対15だった。
2.The adult to child ratio is 1 to 6.
成人と児童の割合は1対6である。
3.What is the ratio of men to women in the department？
この部門の男女比はいくらですか。
4.The ratio of pupils to teachers was 30 to 1.
生徒と教師の比率は30対1.
5である.The ratio between the two sides is one to four.