Is it ensure, insure, or assure?
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.
ratio ra·tio (rā'shō, rā'shē-ō')
n. pl. ra·tios
Relation in degree or number between two similar things.
The relation between two quantities expressed as the quotient of one divided by the other.
A relationship between two quantities, normally expressed as the quotient of one divided by the other. For example, if a box contains six red marbles and four blue marbles, the ratio of red marbles to blue marbles is 6 to 4, also written 6:4. A ratio can also be expressed as a decimal or percentage.
An expression of the relative size of two numbers by showing one divided by the other.