When two variable quantities have a constant (unchanged) ratio, their relationship is called a direct variation. 
It is said that one variable "varies directly" as the other.  

The constant ratio is called the constant of variation.

The formula for direct variation is y = kx,
 where k is the constant of variation.
"y varies directly as x"
Solving for k: 
(y = numerator; x = denominator)

Example:
     The weekly salary a woman earns, S, varies directly as the number of hours, h, which she works.  Express this relation as a formula.

Answer:    S = hk    or       (where k is the constant)