This interactive activity includes THREE separate work areas. In the first, "Shopping", you can weigh food items and answer questions about the unit rate and about the cost of different quantities of these food items. A double number line (cost and weight) is there to help you with the rates.
The second work area is "Shopping Lab". In it, you can decide the unit price for apples, carrots, and candy, and you can explore the cost of different quantities of these items using the scale and a double number line.
In the last work area, "Racing Lab", you set the speeds for one or two different cars and then make them race, observing the double number line (miles and hours).
These activities fit best 6th and 7th grade math.
Credit: This activity is created by PhET
Screenshot from the activity: