For a big cleaning job, Manny wants to mix some ammonia-based cleaners to get a certain concentration of ammonia. He has 5 litres of cleaner that is 20% ammonia. He also has a large supply of cleaner that contains 10% ammonia. How many litres of the 10%-ammonia cleaner does Manny need to add to the 20%-ammonia cleaner to obtain a batch that contains 15% ammonia? Write your answer as a whole number or as a decimal rounded to the nearest tenth. litres