Answer:
To determine how much of the job they do together in 1 hour, we can use the concept of "work rates":
- Isaac's work rate alone = 1 car / 7 hours = 1/7 car per hour
- Isaac and his dad's work rate together = 1 car / 1.5 hours = 2/3 car per hour (since they finish 1 car in 1.5 hours)
By adding their work rates together, we get the work rate for both of them working together:
1/7 + Dad's work rate = 2/3
Subtracting Isaac's work rate from both sides gives us Dad's work rate:
Dad's work rate = 2/3 - 1/7 = 12/21 - 3/21 = 9/21 = 3/7 car per hour
This means that Isaac's dad can wash the car by himself in 7/3 hours, which simplifies to 2.33 hours or 2 hours and 20 minutes.
