Andy and Zack have to sell $$1200$$ aircrafts for a new marketing campaign. Zack can sell $$150$$ aircrafts per hour. If Andy and Zack work together, they can sell all the aircrafts in $$6$$ hours. How long would it take for Andy to sell all the aircrafts by himself? 
Overseas Competition->Knowledge Point->Word Problem Modules->Work Word Problems->Collaborative Work Word Problems