A volatile organic compound (VOC) is to be adsorbed from air onto a bed of activated carbon. The equilibrium capacity of activated carbon at the feed conditions is 0.4 grams VOC per gram of activated carbon. The column contains 4 grams of activated carbon per cm² of cross-section. The feed rate into the adsorber column is 0.2 grams VOC cm⁻² h⁻¹. Breakthrough time is defined as the time at which the concentration at the exit of the bed (c) reaches a value of 0.05c₀, where c₀ is the concentration of VOC in the feed. The breakthrough time for the bed is 2.1 h. The area under c/c₀ curve between the initial and breakthrough times is 0.1 h. Which one of the following is the fraction of unused bed at breakthrough?