A 56-year-old woman with a 75-pack-year history of smoking cigarettes visits her physician in Mumbai because of shoness of breath. The physician sends her to a pulmonary function laboratory for a complete workup. The table below shows pulmonary volumes and capacities obtained using simple spirometry and helium washout techniques. Lung volumes Amount Functional residual capacity 4.5L Inspiratory reserve volume 1.5L Inspiratory capacity 2.0L Vital capacity 3.0L What is the residual volume of this patient?