A proportional relationship grows (or shrinks) at the same rate.
Examples: If a basketball team scores 15 points in the first quarter, then a proportional relationship predicts that they score 15 points in each quarter, so they would have 30 points by half-time, 45 points at the end of the third quarter, and 60 points at the end of the game.
Another example: If you make $10.00 per hour, then after two hours, you have $20.00; after 6 hours, you have $60.00; and after 20 hours, you have $120.00.
Example of relationships that is not proportional:
Human growth rate: as a baby, you grow by the greatest amount of increase, as you get older, your growth rate slows, and stops.
