Charles can do the whole job in an hour, Bill can do 1/2 of the job in an hour, and Bob does 1/3 of the job in an hour. Therefore, they do a total of 1+1/2+1/3=11/6 of the job in an hour, meaning it takes them 6/11 of an hour. This is 6/11*60=360/11 minutes, or approximately 33 minutes.
