John, Jonah and Angus take $$27$$ hours to build a brick wall. Jonah and Angus (without John), working just as hard as each other, together take $$36$$ hours to build the same wall. How long would John take to build the wall on his own? 
Overseas Competition->Knowledge Point->Word Problem Modules->Work Word Problems->Collaborative Work Word Problems->Finding the Personal Productivity Given the Productivity of the Team