An empty tank needs to be filled with water using three pipes A, B and C. Pipes A and B alone can fill the tank in 16 hours and 20 hours, respectively. If the pipes A and B are opened in alternate hours along with C, i.e., in the first hour, pipes A and B fill the tank, in the second hour pipes B and C fill the tank and so on, then 90% of the tank is filled in 11 hours. How long, in hours, will pipe C alone take to fill the tank?