Graph Proportional Relationships Practice Test

A bike rental shop charges a constant rate. The relationship between hours rented $x$ and total cost $y$ is $y=9x$.

How much does it cost to rent a bike for 4 hours?

A science experiment uses a mixture with a constant ratio: for every 1 cup of water, 7 drops of dye are added. Let $x$ be cups of water and $y$ be drops of dye. What does the slope (unit rate) mean in this situation?