You’ve probably seen graphs with direct variation in a number of examples. Perhaps you’ve seen a graph of earnings expressed in dollars or the water level in a bathtub as a function of time. You’ve even looked at a graph of the temperature of a cake in relation to the time it takes to bake it. You can quickly determine if a function is direct variation when the x and y values are both zero. If not, try dividing y by x.

## Graph between earnings in dollars and hours worked

This graph shows earnings in dollars per hour worked for a variety of professions. Doctors, for example, work around 54 hours a week and make an average of $84.48 an hour. On the other hand, the least productive profession is “Graders and Sorters of Agricultural Products,” with an average annual wage of $19,320. For both professions, the difference between hours worked and annual pay is about a third.

This chart shows the average number of hours worked across countries, and it shows that national income has become strongly related to working hours. The trend is positive, and it makes sense that the link between earnings and working hours has increased. In the early industrialized countries, people worked more than 3,000 hours per year, or about 60-70 hours a week for 50 weeks a year. However, the working hours of these countries have been decreasing for a century and a half.

## Graph between water level in bathtub as a function of time

A graph between the water level in a bathtub and time is a useful tool for predicting how long it will take a bathtub to drain. For example, if a person uses a tub with a shallow bottom, the water will drain faster if the water level rises above the bottom. It is also helpful to know the amount of water that is left in the tub at different times of day.

A recirculation circuit’s set water level data is generally altered if the connection position of the bathtub 1 with the recirculation circuit 14 deviates from the value in the piping construction drawing. This means that the level of water in the tub will drop in increments of five cm.

A reference water level calculation unit (30) calculates the amount of water accumulated until the first hot water circulation. This amount is called the standard water level. This amount is obtained by multiplying -bK by the size of the bathtub. The reference water level, or the’reference water level,’ is the height measured from the bottom of the bathtub to the center of the circulation port.