Actually, there's insufficient information. As washing machines go it seems that 4.5 cu ft will manage 20 lb of laundry, but if we're to fill the washing machine to full capacity it probably wouldn't work, because there must be room for water, or the clothes would not be washed properly. There needs to be space for the clothes to move about in the water and the water level doesn't come up to the top of the drum, allowing the water to move about inside the laundry. There's no direct conversion, but if we use proportionality, 2.9 cu ft would be equivalent to 2.9*4.5*20 lb=12.9 lb. However, it's probably true that this amount of laundry fills only about half the space available (the water has to occupy the space, too!).