Inversely proportional

Inversely proportional means that one variable goes up while the other goes down.


3 workers paint a house in 12 hours. How long would it have taken for 9 equally productive workers?


If Y is inversely proportional to X, the equation is of the form:

Y•X = k      (where k is a constant).          Y=k/X

Ratio Relationship of distance=speed·time

Speed and time are inversely proportional because as the speed increases, the time it takes to reach the destination decreases.


David travels 120 miles at 40 mph. How long does it take him?

DST table

Problem. If you were traveling 60 miles per hour, how far would you travel in one hour? Two hours? Three hours?   Make a table and a chart showing the number of hours traveled vs. the number of miles traveled.   How many hours did he travel at 45 miles per hour? 


Distance Speed Time Problems

Inversely proportional Word problems


Example: Ice Cream Sales 


Ice cream demand rises with temperature. When it gets too cold, sales go down.

PROBLEM:  You want to determine the relationship between sales and temperature.

Example: Blanket Sales