Pipe A and B can fill a tank in 10 , 20 respectively while C can empty it in 30 hours. They are opened in alternate hours like A is opened in first hour, B in second hour and C in third hour. But just before the start of the work some dust enters in pipe A and B which reduce their efficiency like A works with 50% of his efficiency, B with 2/3 of his efficiency. Find the time taken to complete 3/5 of the whole work.