Question: If Raymond does half as much laundry as Sarah, and Sarah does 4 times as much laundry as David, calculate the difference in the amount of laundry Raymond and David do if Sarah does 400 pounds of laundry.
Answer: 
If Sarah does 400 pounds of laundry, and Raymond does half as much laundry as Sarah, Raymond does 1/2*400 = 200 pounds of laundry.
Sara also does 4 times as much laundry as David, meaning David does 400/4 = 100 pounds of laundry.
The difference in the amount of laundry Raymond and David do is 200-100 = 100 pounds