Variation problems often set up as rational equations. Given two quantities x and y the following keywords indicate a particular relationship where k is called the variation constant.
Example: Weight on Earth varies directly with the weight on the Moon. With his equipment, an astronaut weighs 360 pounds on earth but only 60 pounds on the Moon. If another astronaut had landed on the Moon that weighed 54 pounds with her equipment, how much would she weigh on Earth with equipment?